Monsters that do more RP damage than health damage?

I had my party fight a group of Nothics a few weeks ago. The fight was ok, but they forced the players to come up with secrets for their characters that are now with me and might be used in the future.

A monster that deals anough damage to down any character in one hit? Party smiles at the challenge.

A monster that can damage the party's equipment with acid or rust? Party starts to pray.

Hags, Nothics, most Fiends. Mostly what you’re gonna be looking for in terms of things like this would be a hag.

There's gotta be some conversation beforehand, definite Session Zero topic.

I played a Bugbear a few years ago and he got a little too close to a Fireball. Dex Saves and Rogue class abilities meant I only took 3 damage, so we made it that it burnt all the hair off his rear end (he's allergic to pants) Bugbear-ass jokes and 'Babooning It' became regular bits.

Some examples I can think are flumphs, rust monsters, vampires, werewolves, and just about any disease carrying creature. Possession if done right could also be fun for players to RP.

If you're into these kind of effects, check out the madness tables in chapter 8 and the lingering injuries table in chapter 9 of the DMG. I'd recommend collaborating with your players before inflicting these effects and give them a clear path to cure them.

Many moons ago i made a post about "using nobles' as antagonists.
I feel like they would fit the description for this, but also any monster with their "features".

Any monster that, is part of an organised group ( liches, spellcasters, fiends, fey,) that might seek revenge for the fallen one, or even ressurrect them.

Any monster that is not killed by normal means ( liches, fiends, fey, elementals, and other extraplanars usualy get banished back to their plane). A noble usualy has the gold to have some ally cast ressurrection, or have powerfull wizards cast spells like "death ward", or "contingency".

Any monster with vast amounts of minions, and influence that can be felt before, during and even after the fight.
You killed their boss, ok, but now they are pissed and you still need to get out of here alive.

Some people are attached to their characters and may not react well to physical changes that come off as forced or unearned. I once had a DM significantly age certain players characters via some evil magic by like 30 years. I don't think they enjoyed that at all.

Thats almost like killing the character they wanted to play with one dice roll.

Something "fixable" is okay, something permanent is not. Unless it was understood that it can and would happen in a session 0.

My two cents.

Strahd can totally do this, if you run him a certain way. He doesn't even need to use magic or his vampire powers, he can just lie and manipulate.

When I ran a campaign set in Faerie, I made it so that fairies did not have a use for coins or any kind of money. But you could trade with them using other things, such as precious memories. The warlock lost his memory of the years he spent being the monk's road trip buddy. Others lost language or tool proficiencies, and one temporarily lost a skill proficiency (which would be reverted after a week; the other losses were permanent).

It’s not a bad idea in theory but there’s a lotta that in old editions with ability/level drain, dismemberment, alignment shifts, curses (mummy rot my beloved), and longer term poisons. Those rules can be a decent basis for other effects, scale of issue notwithstanding.

A couple systems use “Sanity” or some similar numerical score to represent neural atrophy.

It’s totally something you can do, to have monster attacks leave evidence of conflict on the party members and they don’t need to have big or even statistical representations on a char sheet. It can drive character development, it can be something NPCs talk about, can be something for PCs to chat with each other about, and so on. “Cool scar bro”, ect.

BUT, if you are interested in numbers, then making bruises and stuff reduce max HP by 1-2 points and heal after a number of long rests is totally viable. Sicknesses that cause exhaustion levels, poisons which stick around forever, mild curses that cause small hallucinations, ect. Status effects lead party members towards side quests to remove those status effects, and it all spirals from there.

I’m surprised more people aren’t bringing up lycanthropy. Get bit by the werewhatever and fail your Con save, and now you have to think about how your character will respond to the social stigma that lycanthropes might have, the experience of transformations, and the within-world possibility of a cure.

I don't know if this counts, but my DM has a Devil who occasionally appears. He loves to play games of chance, but only bets for RP related things. He's bet for things like increasing how old you are, a Noble character's prestigious name, a Bard's ability to feel music.

Most recently, my character bet his ability to feel romantic love against the location of his imprisoned former fiancé. It ended up as a draw, and they traded, so now my char knows where to go to rescue her, but when he gets there, he can't feel any love for her. The stakes feel so much higher than if it was gold or items

I had a Deva in disguise at one point and a Kobold Warlock (who have sunlight sensitivity) cast Dispel Magic from 5 feet away. Poor thing was blinded for an entire day after having THE ESSENCE OF SUNLIGHT SPILLED INTO HIS EYEBALLS. Fortunately, the battle was a misunderstanding and ended the turn afterward but still I did levy a (semi)permanent debuff on that Kobold for recklessly burning his eyeballs out.

I'm actually writing a SuperHero Campaign where I split HP out.

So after calculating their health, 25% of it is actually HP, 75% of it is designated as Heroic Toughness (or just Toughness for Short). So a Barbarian with +4 Cons would have 16 HP, but 4 HP would be HP, and 12 of it would be TP.

The Toughness Points showcase your heroic willpower to power through attacks, and it's only when you've gone through your Heroic Toughness that you actually start taking damage. To ensure that they take care, I will instill actual injuries whenever they take damage into their actual HP. Normal civilians will not have TP, so even someone who would be like a Barbarian (like maybe a Weightlifter) would just have that 4 HP. So it behooves our players to protect civilians since they have so little HP and even taking 1 HP could render them with a permanent or semi-permanent injury.

The Noble stat block is a pushover as soon as you reach level 5. But pissing off the wrong noble can spell disaster whenever you're around settlements that they rule or can affect.

“RP Damage,” as in a monster that negatively impacts roleplay? I mean there are plenty of monsters who have capabilities to “negatively”and/or permanently affect a PC’s circumstances. Vampires, werewolves, mindflayers exc.

The only time it would negatively affect RP is if either: A. The player doesn’t want to lean into RPing it, or B. The DM didn’t account for it.

A whole campaign is built on positive, neutral and negative outcomes for roleplaying…

If stink is what you’re going for, the Otyugh is certainly a top contender for stinkers. A monster that dwells within trash heaps and junk yards (think the sewage monster from Star Wars) with a diseased bite and limited telepathy

Use conditions.

Afflictions or curses that can be removed, or even better, disappear after a day/long rest.

You can use these afflictions (described in whatever way you want) to do things like grant disadvantage or minus penalties to certain RP checks like persuasion or investigation or you can have it so they must make a charisma saving throw whenever talking to someone to prevent them from being laughed at. Whatever you can think of.

Maybe not exactly what you are asking for but: Any noble from your world. Attacking, killing, or crossing them should/could have severe consequences such as getting declared an outlaw, having mercenaries sent after you, getting banished from the land, having all your assets seized, having your friends and family harassed or killed, etc.

A mimic infestation.

For weeks afterward, they thought everything was a threat.

What about a character that instead of dealing HP damage, it deletes coins from your inventory?

Troglodytes are what you want. They have an ability called Stench:

Stench. Any creature other than a troglodyte that starts its turn within 5 feet of the troglodyte must succeed on a DC 12 Constitution saving throw or be poisoned until the start of the creature's next turn. On a successful saving throw, the creature is immune to the stench of all troglodytes for 1 hour.

Play it as the trog plasters the player with some kind of goo if they fail the save. Once they pass the save the player is acclimated to the stench so they can’t smell it on themselves. But all NPCs reacted somewhere between forced politeness or gagging in the streets.

My players haven't met this yet, but I have some marionettes that are enchanted. They use illusion magic to copy your worst fear and haunt you while in the dungeon. Otherwise, they just look like those creepy doll puppets from the Victorian Era. Which is bad enough by itself for a lasting impression tbh.

I actually really like this idea and was already thinking about how gore and guts should make them kinda disgusting and hard to talk to. I think a temporary effect after fighting a boss is a fun way of making boss fights stay with a party for a little while. I think if you can test the waters by making them deal with something that disappears during their travel to and from a dungeon and then slowly roll out more stuff.

One of my players was hit with the memory eater ability that elder oblex have. He wanted to make the effects permanent for character building purposes.

Not really RP but not HP Damage, Rust Mosnters and Slimes usually eat through equipment

some rust or acid monster that dissolves equipment and clothes? you need to rp a bunch of naked adventurers in a dungeon.

Any intelligent monster who gifts the party a very high level magic item that they are all going to want. Just one.

Quest to find the party's favorite NPCs cat led them into the sewers. Once there they found the cat stuck in an entirely too small cardboard box it had sat in as cats do. It was at the back end of a large room beneath the city's opera house and the box was stuck in some poo. I had intended this to be an otyughs home, one the city tamed and trained to clean the sewers as it could eat everyone's... disposals. Anywho I made it more fun and had it instead be the home of the Great Mighty Poo from the N64 game Conkers Bad Fur Day, complete with operatic introduction. I used a homebrew slime king stat block I found and had it summon small scat elementals every time it launched an attack. Party ended up unable to beat it and had to cast sanctuary, have someone race for the cat and flee which was awesome.

The party stunk horribly and had a fun RP session at the city bathhouse where they had their comical and stereotypical anime bathhouse episode.

Honestly, I think it's a cool idea. They're the sort of enemies I'd pepper in as weak support units for actual enemies. Like a sick wolf that is part of a pack, when that specific wolf tries to attack a player, it vomits instead giving making the player very unpleasant smelling until they have a wash, maybe it could give the poisoned condition to for the actual fight.

The only problem is I can't really think of any good examples other than poison and disease related. Maybe injuries, too, like you mentioned. But injuries are tough to implement right. Maybe like an enemy with a hot brand or a whip, not intending to kill but to cause pain and injury while his buddies do the killing. Like those enemies could cause temporary scaring until it's treated.

I'd say anything can have an effect if done right, and if players are the type who lean into the roleplay. Having your clothes bloodied can certainly affect the way, say, guards are going to pay attention to the new faces in town. Do your players take time during downtime to freshen up, maybe bathe in the river? Someone also mentioned that illnesses, poisons and possessions can be fun fodder for "affecting the pcs after a fight" and I agree. As long as you don't force permanent changes to the pcs you should be fine.

You could have some sort of thing that when killed it haunts the player for awhile. So the fight seems easy until the first one goes down and still seems to be there.

Make them all look identical like cursed dolls or something. Then have them just show up randomly and only be visible to the player who killed them.

To really make them question their sanity have several of them get away in the fight or imply that more exist so when a player sees one they don't know if it's them being haunted or one of the real ones coming to try and finish the job.

https://youtu.be/8ybVJZR-5QI?si=_0UcMWFbT1pVRKk0

This is a really cool video about spells that have this kind of impact

rust monsters and shadows

I remember pathfinder linwyrms (or something close I forget the spelling.) they were unique in that the act of killing one placed a curse on whoever dealt to e final blow. The curse was based on the species of the wyrm. Many were deeply mechanical, but a similar mechanic could be imposed on far more whimsical curses. Perhaps a fey critter that if killed imparted some bizarrely random curse. Like the inability to see dogs or forcing them to follow through on any offer they make.

Deceptive or mind altering creatures can also have a lasting impact on RP if it fosters mistrust: in others or themselves

Shadows are purely this, and my favorite monster to do in swarms. As long as they can hit consistently, they hurt no matter what level you are. With some tweaking, you could even make the strength drain harder to cure for dramatic effect.

There was an amazing... cartoon, probably? about a fae monster than ate letters from the characters names, and kills them when they run out. Then they run into some NPC who's called Harvey Rumplestiltskin von Finkenstein who says that his parents hunted them, and the party are like "clearly!"

Rakshasas!

Curse you with nightmares that block long rests if you get hit by their claw attacks.

Absolutely has no bearing on the encounter itself, but can have a huge impact afterwards-

(Kuba Kenta curse arc from JRWI comes to mind as a good example)

Edit: Sorry. After reading a little more carefully, I realized you meant nonviolent damage to the PCs, not the players.

I offered to run one on one sessions for my players during Christmas break since scheduling can be tough.

I gave my player who LOVES RP and isn't that into combat a full session of helping a showgirl satyr finally ask out her minotaur bouncer crush in order for him to sneak past security and rescue another PC.

He seemed to love it, but before the next group session a couple of them were talking about the things they had to do and he yelled, "All you had to do was fight a horrible monster by yourself! I had to deal with relationship problems!"

I asked if it was really that bad and he shot back, "That minotaur is like three times her size! Will she even be okay?!?"

After enough time had passed for everyone to forget about the whole thing, I sent the party an invitation to her wedding. 🦌+ 🐂 = 💕

Nothic can learn something about a character, if that gets shared it could be devistating

False Hydra.

Companions, people, adventures that never were, forever forgotten like they never existed.

Hell, you could introduce a whole beloved character that traveled and helped the whole length of the way that the PCs have forgotten that was scrubbed from their memory after the encounter, with only the pit of where they used to be as the only thing left behind.

The mental damage that having an entire part of your life ripped away would linger for quite a few sessions in an intangible, tangible way.

Oh man once i did a quest where a player got separared from the group and he found an intelecet devourer.

easy combat, right?

but as the intellect devourer hits, I told the player that the creature "ate" one of his memories the face of his late mom.

the player was a monk urchin, it was a very emotional moment for him. Great rp after this fight.

Do PCs really have a "body" outside of HP that you can "damage?"

Treasure, as others say. Titles, lands, relationships... but these things don't have mechanics. Something that impacts Diplomacy checks, maybe. Or something that undermines the party's social standing.

Like a Mummy & Mummy Rot, or just the Lycanthropy Curse from Lycanthropes?

Winter hags eat memories. And mind flayers eat brains. We lost a party member because even though they still had HP, a mind flayer devoured their actual brain.

My young dex fighter got additional 40 years from a ghost
Funnily enough, I started rolling dex better after that, effectively turning into Palpatin

Ever heard of Dread? No health points. No character sheet (not in a number-heavy way anyways, just description). You pull from a jenga tower when you take risks. You can refuse and suffer a handicap (get sick, lost, slowed down...). If you pull and it falls, your character dies.

So yeah, I think there's a lot of room for effects to not even touch our healthpoints yet feel very significant.

Oblex and spawn, doppelgangers

I say that your baddies always deal HP damage but only sometimes deal Status damage. Imagine you are thrown against a wall and fail DEX (Acrobatics) to save yourself, you'd get dealt 1d8 or so and gain disadvantage on all STR (Athletics) checks/saves due to a cracked spine.

Sometimes, if the situation calls for it, I will let you substitute some painful hit in exchange for a nasty Status move. Imagine you are surrounded on all sides, restrained at the legs, and they ready up a 3d20 Hammer Swing. I will let you hold up your arms in an X formation, shattering your arms and rendering them unusable in exchange for only suffering 1d20 + 6 or so and being knocked prone

This is just spitballing, but it could be fun

My favorite “RP damage” monsters are those that have feeblemind or feeblemind-like effects and having to live with that for a certain amount of time.

at least in my table, it lead to something like the whole party banding together to defend their friend who used to do all the defending themselves.

I just DM'd a session where a hit dealt health and strength reduction. Their health could be restored by normal means but strength required a long rest. Until that happened, they also had to do a saving throw prior to each check. On a fail, they were required to roll with DA.

basically hags

My Owlin dropped to 0 over a large wine barrel during a combat. Now his feathers are stained pink. If somebody tries to remove it with magic Im going to tell them it didnt work lol.

Aboleth for its body horror of mutating characters so they can only breathe water. Not to mention the hideous reveal of using its lair actions to deceive the players initially.

A friend of mine created a little green rabbit with dollar signs on it...dc 11 will save or become obsessed with the rabbit, willing to kill to keep the rabbit... It's not a high dc....it has wiped out 3 parties.

Our dm threw three of these at us. They rolled awfully and got destroyed. The dm was audibly disappointed and it's a secret heavy campaign, so we just joked about how two more appear.

In an unusual move from our dm, they asked if we would be okay with this, because they were excited to use these against us for RP reasons.

Two more came, rolled poorly and were swiftly destroyed. Another came and met the same fate.

One more came and the PCs all just broke character and arbitrarily chose our artificer as the victim of the nothic. We ran away and just took the dodge action whilst the artificer 1 v 1'ed the final nothic. What happened next was a long stream of misses and poor rolls from both the artificer and the nothic, until 6 rounds later the arti finally did enough chip damage to take the nothic down.

Of course, we were all laughing throughout this. The artificer was totally fine :-)

I believe wraiths or specters (some ghost monster?) has a save that if you fail by under 5 I believe you age 1d4 decades. Greater restoration within 24 hrs reverses it, otherwise is permanent.

There is the obvious mind control option.

It doesn't have to be "DM now controls your character" level stuff, but maybe the PCs each now have a "quirk."  Must boil food, alcoholic, anxiety attacks, confuses gender, can only talk to someone when facing away from them, whatever you (or they) want to think of.

There's a really cool one in one of the Kobold Press books, I'm sorry I don't remember the name (I think it has witch in it). I think the ability is something like Strands of Fate.

Basically, if it hits someone you have the party privately tell you how and at what level any the party member to their left will die. If they guess one part of it true, that member gets a free resurrection. If they guess the second part true (I forget which part is which), they come back with the knowledge of someone importants fate.

Fey are good for this with the tricky deals no?

Not an answer to your question, but have you checked out the ttrpg, Crown and Skull. The game does away with bit points (for the players) and instead ties "health" to your skills and equipment. So when you take "damage," you lose skills and equipment in a very RP damage type of way.

If that sounds slightly interesting, check out the free players guide for Crown and Skull from Runehammer games :).

Maybe not the exact idea you're going for, but ghosts.

Ghosts can possess a player, taking total control of them without harming them at all.  It is a horrific experience.  The character even perceives what is happening but is totally helpless.

Oh, and it lasts as long as the ghost maintains concentration without any other saves. Warning: this takes away player agency. Some players will not find this fun. Only use if you know it will be well received.

Rust Monster is the classic. Eats up your weapons and armor and then scurries away, leaving you with rusty junk.

Rust Monsters, Disenchanters, Ring Worms, Pixies pulling pranks. Bookworms for your wizard. Lhiannan Shee for your bard. Huecuva to mess with your divine casters & give them an idea what happens when they get heretical. Don't be afraid of cursed magic items.

Any monster with Feeblemind

Rust Monsters and some slimes deal damage to normal weapons, armor and shields that can be a factor in dungeons where you are whittling at a parties resources making them decide if its worth it to press on or try to turn back and repair their equipment but risk the dungeon repopulating or getting plundered while they are gone

my party is fighting a hag with a warlock minion. that warlock is the gnome bard's teenage sister. they just found out about their pact last session when they showed up to save the sister from "being kidnapped"— turns out she wasn't kidnapped, she ran away. the whole situation is gonna emotionally hurt the gnome bard for the rest of the campaign probably lmao

There's a Wraith from Ravnica that can cause indefinite madness.

I don't remember who said buut having a regular monster like a slime make the character significcatively age old could have great rp implications

I was the only serious roleplayer at a table of murderhobos. I received both blessings and curses as a result. I always rolled with it, because it was like he was making my dedication for staying in character reflect what was actually happening.

For context, we were playing 5e, multiclassing wasn't wise from a gameplay perspective but I was committed to taking a level in cleric while otherwise being a draconic origin sorcerer. A half elf cleric of Hannnali Celanil, elven goddess of love and beauty, to be precise. And my character was all about the aesthetic, about worshipping beauty at all times, about physical sensations, you get the idea.

A showdown at the tavern happened, and the NPC monk stripped down in preparation for battle. DM had me roll for concentration, and if I failed I would get disadvantage on everything for a round "because you're distracted by Dat Ass". This ruling was beloved by EVERYONE at the table.

As a sorcerer character who loved and was obsessed with his own magic, which I frequently added things to during downtime, I was memorably once slapped by a beholder's anti magic field and was given such a great description of how I all of a sudden felt wrong and empty, and when I called for a reassuring spark of lightning in my palm I felt nothing, and I had the option of rolling either a Charisma or Wisdom save to not run away in a screaming panic. How lovely it was to have something I had invested so much time in portraying to pay off, even if it was something negative!

So I guess my advice is that if you want to hit a player hard, hit them where it hurts, and you don't have to ask a roleplayer where it hurts, because they've been transmitting it the whole time.

Pathfinders erodaemon's ability is tranposable to 5e. TLDR, they have constant detect thoughts, and can use it to sense a person's "object of desire", sensing the person that PC desires to see most and transform into that form. It doesn't have to be lust, in fact they usually take the forms that play into heartbreak and despair. It can be someone they miss dearly or a lost loved one.

They can kiss someone (doesn't have to be someone who adores them but the ability works better if the subject is tricked, also can be reflavored away from kiss for certain tables). If successful, the subject takes damage if they are away from the erodaemon. They try and lure a PC in by reading their minds to learn about the loved one and pretend to be them, draw the person in and then make it so they cannot leave, but why would they want to leave their loved one?

Fomorians can disfigure people by looking at them which leads to penalties with Charisma and movement

I once used the Treacle from Kobold Press in a 5e homebrew I was running. It's sort of a lower psychic ooze that shapeshifts into pathetic forms it intrinsically gleans psychically from its target. It has NO attacks! Its shapeshifting is dependant upon its size category. Thus, one with a small mass could mimic an injured small animal or even a baby while a medium one might mimic a suffering in pain old lady who cant walk. The insidious bit is that it causes light damage on contact, slowly leaching blood from creatures who come in contact with it.

Had the party on the lookout for missing children, they find a cave chamber with a few dead kids and two seemingly alive. One is a semi-catatonic girl gripping a teddy bear (the bear is ooze 1) and the other is an injured boy, crying and cradling a broken arm (ooze 2). They rush the kids back town with the intention to get them medical help and then they head back to the caves to continue exploring. The only clue was that the pally noticed he was covered in blood after dropping the kids off (with a mysterious loss of negligible HP with it). Anyways, they find more children and finally come back to town, which is under a quarantined lockdown due to a mysterious plague.

Their faces when they realized they release the oozes into the villagers midst......muwhahaha

You just gave me a brilliant idea:

A fungal species that causes an insidious form of encephalitis. A creature with weak stats, but on death releases a cloud of spores, and everybody who is in range must make a constitution saving throw, but you don't say who succeeds and fails, you just jot down the results.

Over the next few in game days, weeks, or sessions, you start to impart secret modifiers to the players who fail. It begins with mild anosmia and other perceptive changes, they receive a -1 to any perception checks. Next, their somatic sensory and motor cortices are hit, And the fungus grows stronger, increasing the modifier on all checks to -2 and now including all dexterity and strength based skill checks, including attacks. As it continues to grow and spread, enough of the brain is affected that their reasoning and judgement is impaired, resulting in all intelligence, wisdom, charisma checks, and any spell or other modifiers relying on these stats, being impaired as well, and the modifier increases to -3. Finally, as the fungus fully takes root, whenever the player exerts themselves in any of these ways, whether attacking, singing to perform, or even just trying to remember the name of an old friend, they roll an additional d4. On a 1, they begin to seize, becoming stunned and fully incapacitated for 1d4 rounds, after which they lose consciousness altogether.

To cure this, a powerful spell or antifungals must be acquired to heal the player, or, after the 10th seizure, they will die instantly.

Sometimes it doesn't even have to be a monster that does this. I once had a party trekking through a swamp, and one player failed a Dex save and got their boots filled with mud. This was the Halfling, so extra funny. If he had taken 3p of damage, it would have been forgotten, but the soaked boots were referenced quite a few times after this.

Not sure if it's quite what you're after, but I find the death dog to be super underused as a monster. It has two bite attacks and if hit, the target must make a Con save or be poisoned (disadvantage on attack rolls and ability checks) until the disease is cured, plus every 24 hours after they have to save again or reduce their hit point MAXIMUM by 1d10.

The con save isn't very high so it's not a guaranteed kill, but long-term poisoning is nasty and if they're not expecting it characters might not have Lesser Restoration ready to go, and the look of absolute terror on a party's face when you tell them to reduce their hit point maximum on the second roll is truly something to be treasured. If you make sure to describe the disease as festering, black, noxious, and resistant to normal healing it can continue to impact the players long after the initial encounter.

Plus the death dog is only CR 1 and doesn't scream 'boss monster', so you can have a couple of them spread across an adventure if you want to be particularly nasty.

Rust monster.

Troglodytes.

Gay Ooze (Gray ooze, but I refuse to change the autocorrect)

Giant skunk.

Poison ivy. Just regular poison ivy

Have them fight a giant skunk ;).

Awesome.  I love the idea of lasting effects.  I can't think of any that are spelled out so you'll have to make some things up. Just let players know ahead of time,  either at game start or at first time it comes up, that you're doing this. You could even do this with general adventuring.  Have NPCs comment about their appearance or smell. In the game Kingdom Come: Deliverance, people will comment when your clothes are torn or dirty. 

The closest thing in any of the books are probably diseases and curses. again, I don't have a extensive list in my head, but the mummy comes to mind. Just add the effects as diseases or curses progress.

 One of my house rules is that max HP or stat drains don't recover fully after a rest. Rests are so easy so it rarely matters.  So for example, the players get lost max hp back at one hit die per long rest.

Nothics can be fun due to their Weird Insight ability. On a failed save, they learn a secret about their target. You can cause all sorts of chaos both by getting critical information into the hands of your BBEG, or by revealing said secrets amongst the player characters if your party enjoys in-character conflict amongst themselves.

In 3.5, shadows used to attack your Strength score (don't know if they still do). What you're talking about is basically Charisma burn. You could have the monster or monsters deal 1d4 damage to a player's Charisma bonus, with each 2 points of damage coming with a unique effect.

For example:

• 1-2 damage: The character has a noticeable stench of death about them, resulting in a -1 penalty to Charisma-based skills.
• 3-4 damage: The character has weeping wounds on their body around their face and hands, resulting in a -2 penalty to Charisma-based skills.
• 5-6 damage: The character has an obnoxious laugh and tone of voice, resulting in a -3 penalty to Charisma-based skills.
• 7-8 damage: The character has an unsavory aura about them, resulting in a -4 penalty to Charisma-based skills.
• 9-10 damage: The character loses the ability to understand social cues, interjects assistance on nearby Charisma-based checks, resulting in a -5 penalty to Charisma-based skills and disadvantage on all Charisma-based skills.

The effect can last for 1 day per tier, reducing down a tier every day.

Monsters that have non-lethal poisons: blindness, confusion, deafness, drunkenness, memory loss, weakness

Monsters that spray you with an attractant. Attracts (pest, other monsters) within "x" distance for (hours, days, weeks)

Monsters that trigger allergies. Coughing, sneezing, itching, runny nose, itchy eyes, watering eyes.

Thief Monsters. They steal something off your person and run for it. May be especially aggravating in swarms. You may be able to find your stuff in their nest with may be difficult to find or difficult to get to.

Succubus/ incubus can do this really well. They're not good fighters, decent at escape, great at seduction. Not appropriate for every table.

Lamias can enslave entire villages. My party is about to fight one and she managed to get a lock of hair from one of the players without them knowing.

The constant wisdom saves due to Scrying drives them absolutely crazy because they don’t know what causes them. They also don’t understand how the unknown evil can always be one step ahead of them. Its hilarious.

Include an engraved locket on the corpse of an orc they kill, "world's best dad" with pictures of his family, also a note in common "we'll miss you daddy, come home safe"

Slaadi in Low level are this Horror to the max.
They dont intend on killing you, but if you survive congrats you are now pregnant.

Nilbog... a monster that hurts you if you attack it and is basically an incarnation of chaotic pranks.

Dire skunk

Aboleths are a big case of this; pretty underwhelming in combat, but that disease can fuck you up if you don't have the means to cure it.

I ran a false hydra as a plain monster. It didn't kick in to the player after theybdefeated the monster just how much damage the monster did. NPCs they remember no longer existing, Benefactors missing, the tavern closed and abandonned.
It started as a murder mystery, it ended as the most memorable monster my players ever encountered who's impact in the world is still felt today even 300 years into the future canon lore.

We Fought a powerful Mind-Flayer that escaped the fight. During some down time months later, me and the DM figured it jumped me in my home and after a hard fought battle(my character was LV 16 or 17 at the time so a 1v1 was very much feasible) he lost and now has a partial mind control aspect going on, the only reason he isn't fully controlled is because he's a cleric and has a lot of backing from his God(The Forge). So you can make any monster or creature have lasting RP effects if you think of how to use them

Nightwalkers spoil food/water, rot wood, and such within 20 ft of it. Great way to throw a wrench in travel plans if your party has a wagon with supplies.

Honestly rule of cool, there’s no reason you can’t pick a monster like a slime or goblin or something and say that there’s a curse on them which rubs off on/ passes to anyone who touches it. Make stuff up!!

Chaos beasts

I introduced the concepts of Doppelgangers by having a character's long lost "dad" stab him when he went in for a hug. That character is always certain that there is a doppelganger so much that he did stab his dad when he did show up.

A group of goblins with a nilbog among them.

False Hydra

Rust monster. It hits the party where it really hurts, the coin pouch

Rakshasas inflict curse conditions after.

Have bad person kidnap people close to person then later kidnapped person is disguised as an enemy that your party kills unknowingly. Have other kidnapped people sprinkled through the campaign so the party never knows if they are killing a friend or family by accident.

Rust monsters perhaps are a nice in between

Would probably have to be homebrew, but perhaps a monster that invades the mind/dreamscape? I'm thinking something similar to weeping angels from Doctor Who, minus indestructible. Separate the party via a trap and have half the party fight this creature. Perhaps give hints leading up to it to not look it in the eyes, but if a character does, have no noticeable effect.

However, even if they kill it, it implants itself in their minds. They would see it randomly as the adventure continues. This could be how it guarantees food or spawns its next kin. You could either have it induce fear or some other negative affect in certain situations (like giving the fear condition when they are in a fight). You could, instead, have it slowly consume their psyche and will eventually "hatch" from their brain and skull after enough time has passed. This would require the party to seek a way to enter the dreams of the affected individuals and fight the dream version of the monster again. This could be especially cool as maybe it's a bit stronger, but since it is a dream, the host could have a few "token" or something that could allow them to bend reality a bit since it is a dream.

EX: Cast fireball but expend a token to make it lightning instead. I swing with my greataxe but can't quite reach, so I expend a token to give it an extra 5ft reach.

Doesn't have to be a lot of tokens and maybe the other players could get a few too. Definitely would have to be careful with what you allow, but this should be a cool monster and a nice little treat for players to get a little extra that they can do for their turns, even if only for one combat encounter.

The amputater

If you think about the ecology involved this is a large list. Otyughs are garbage eaters that live in cess pits. Swamps are notoriously odious and infested. Troglodytes are notoriously ill odored too. Unless you're wanting weird magical curses. Stink is by far the easiest problem to come across. There are old stats for a skunk too. Not trying to focus exclusively on the Stink here, it is just an easy and specific thing you mentioned.

Not a monster but I've added mechanics of sensory denial to monsters/weapons, like when this monster hits you roll a, do a CHA save, on a fail roll a d4, 1 = blinded 2= deafened 3 = muted 4 = numbed (penalty to physical skills rolls) (If you wanna be fancy you can add remove taste but it's meh) Can make it for a round a curse or until saved, but it is scary to have a creature and be put in the sensory deprivation chamber and being "muted" is surprisingly fun for players to rp or rules lawyer "you cant do x, you couldn't hear them/talk to them/see them"

Have the monster kill the dog

That's RP damage baby 😎👍

Ropers terrify people irl

The Bagman.

One of the worst things I’ve done to one of my payers was have a rodent rip holes in his bed roll (it was a really fancy one). It’s been months of play now (they haven’t been near civilisation since) and he only just started trying to repair it with a needle and thread.

It wasn’t going well… then the Druid mended some ratty fox fur to it to fill the hole. The look on his face as he said “you could have fixed it this WHOLE TIME?”. Priceless…

You could use ghosts. Lots of options there like having them taking on the appearance of loved ones, having them age the party, or having them possess a PC, etc. Their stats should be easy to tweak if the CR is a concern

Black dragons. They are cunning in the most cruel ways. Attacking the party through psychological means and stealthy tactics. Each time they try to sleep dragon will fly by breath attack. He waits for them to reach the thickest muddy parts of the swamp with grasping vines to send his poison dark wielding kobolds. He leaves injured npcs despite for help to use up player spells and resources. When he finally reveals himself, the players be so exhausted from lack of sleep, low on hp, depleted on spells or potions, and best of all emotionally damaged!

having a monster that is essentially a frankenstein of their dead family members…. so many avenues for psychological damage as well!

Doppelgangers are a fun option.

The last episode on the dungeon dudes channel had a really cool fight. The demon they were fighting caused them to age 2d6 years every round. It ended up really striking fear into the cast.

https://youtu.be/rZRZbSIrUaU?t=465

I’ve time-stamped the exact spot that I want you to see

I mean, you could do this with just about any creature. You just have to RP it out. Make it a thing that after a few days of hard living your PCs smell. If they fight a red (or gold) dragon and someone gets caught in their fire breath, singe their clothes. I use the lingering injury rules so gashes, broken bones, bumps and bruises are common place in my game (so is non-magical healing). 

Just let them know ahead of time that these things are going to start happening. 

I have used a night hag to great rp effect. The ability to enter players dreams and turn them into nightmares while also causing issues with rest and health. Very fun.

some sort of slow acting poison/venom damage? your character will be stuck with it after the fight and their health will slowly get worse as it spreads through the body, requiring you to find some sort of cure to fix it before it's too late.

Any of the oozes that can damage equipment come to mind. But they only work up until the items are magic so depends on you as the DM.

I'm writing some random encounters for an enchanted forest that you might like.

Singing mushrooms. Fail the wisdom save and they charm you into spending hours eating them in a daze. You take some poison damage and take a short rest as you recover then wake up with Poisoned condition

Fairy Ring: you stumbled into a fairy ring and they don't like you messing up their yard. If you fail the dex save and mess step on their house they will fallow you around all day tearing at your clothes. Every encounter your Ac reduced by 1d4.

Try fixing the damage and if you roll high enough to make it better and build a pool they will follow you around to mess with your enemies. As a Ba choose a target and lower its Ac by 1d4.

Elder Oblex holy shiiiiit this thing ends up in my Strahd campaigns when I have a party that’s just rolling combat content and I need to show them another kind of fear.

Sibeix, archfey, contracter devil.

Ghosts, prematurely age your character if they spook you on a wis save

had a Party fight a Shambling mound, one of them got Engulfed and covered in "digestive liquid", he was saved but his clothes started to grow mould and moss... he had to throw them away and get new ones but the clothes became a shambling mound-ling and walked off in to the wood whilst he was changing, he lost an item that day

Did you ever play the old Warhammer RPG? I have. And things like sickness from enemies/environments is so a thing there.

I mean mummy rot is cool and all. But what about a creeping rot that slowly turns you undead?

Or a sickness that only affects casters. You aren't sick, your magic is. The Weave is infected. Or your god/patron is. And it got to them through clerics/paladins/warlocks. Or wizards/sorcs.

Personal favorite: brain eating disease. It eats memories until you become a mindless husk. And then the disease is in control. Pseudo zombies, but the usual stuff won't turn them or destroy them. And killing them just leaves a corpse. And makes whoever killed them look like a murderer.

Rust Monsters baby. Narrate armor falling apart as the buckles disappear. Find your way back to town now!