Spell supression zone

I did something similar. At first it was a complete dead spot - actually it was absorbing the arcane energy, so any spell cast was indeed cast, the slot spent, but no visible effect happened.

After a while, it became a gamble - spells worked about 50% of the time, to encourage people to continue casting so the spellsink could continue absorbing juice. It was just a coin toss as to whether or not it worked.