For those interested in expanding the lore and role of druids in their campaigns, here's a Kickstarter that gives you the tools to do exactly that.
My understanding of demi-liches is that they don't pay attention to the mundane world around them until something directly draws their focus back to it. Adventurers entering their home isn't enough, it's more like someone hitting them with a sword is required. So, I don't think the demi-lich will do anything at all. Whatever defenses were in place when the lich moved beyond the standard undead body into its mostly catatonic state while its spirit explores other realms, well, they're still in place and the party will need to deal with them.
It has not been updated for 5th edition. In fact, as I understand it, Castle Greyhawk has not been released outside of Gary's original notes - at least not as it was meant to be. There was a dungeon with the same name released for an earlier edition (whether AD&D or 2nd edition, I can't remember). However, those who played the original say it is nothing like what Gary ran. There is a long, detailed history to Castle Greyhawk which I'm sure someone can relate if you're interested. In any case, the megadungeon exists, though you'd have to develop it on your own.
Calling all druids...Homebrew
For those looking to expand the role and lore of druids in their game, here's a resource you'll definitely be interested in. The project is fully funded, so you'll definitely receive a book
Actually, this bookstore has 7 copies, starting at half the price:
https://www.abebooks.com/9781560761532/Beyond-Moons-Spelljammer-Cloakmaster-Cycle-1560761539/plp
The Marketplace has one copy:
https://www.amazon.com/Beyond-Moons-Spelljammer-Cloakmaster-Cycle/dp/1560761539
For $40, I might be selling my copy soon, too.
The real question is whether or not your deity approves.
If this is something you're not comfortable with -and I can understand why you wouldn't be - then the answer is no. Many people have things they don't want played out in game. Sex scenes, torture, and animal cruelty are a few things I've heard players say are on their "do not include" list. If this is part of your no-go list, the other players should respect it. It's one thing to allude to terrible acts committed by villains opposing a group of heroes. It's another thing entirely to spend significant game time playing out interactions with those villains.
Am I wrong for shooting down a player's request
DnD