Imps are found in the deepest reaches of the game. They are commonly in the Underworld and Hell, they're rarely found in the Crystal Caves.
They cast fire and can levitate, they are weak to swords, and magic, so one or two backstabs from your blade or a couple casts of lightning or magic missile will help you deal with these flying pests with ease.
They can drop spellbooks on their death spot, but most of the time they'll just fall into the voids of the Underworld or the molten lava of Hell.
This enemy has no named variant currently.