The dunce cap inhibits spellcasting (on top of the low intelligence it provides; not specified if it should entirely block spellcasting or just make it very difficult) but also provides magic resistance when worn. This is to add diversity of non-artifact magic resistance builds for non-spellcasters, and also to allow people to give magic resistance to pets that are incapable of wearing cloaks.

Dunce caps are made out of paper. (They continue to be the same color and tile as cornuthaums, though, so it’s not trivial to tell them apart).

When the hero sits in a corner while wearing a dunce cap, give a YAFM and exercise wisdom.