Fire Imp

Fire Imps are an enemy found only in the The Underworld. They shoot three volleys of Burning Sphere which go through tiles, but can be halted by the swing of a weapon (sometimes even dropping hearts). Imps will try to teleport to you if you run away from them far enough that they're offscreen, or after each three Burning Spheres they shoot. Imps can be killed if knocked into lava. Easily one of the most bothersome enemies, it is not uncommon for three to four Fire Imps to teleport to a place where it's impossible to hit them and proceed to fire rapidly, making it difficult to do anything. Try to position yourself somewhere where you can easily deflect their attacks with your weapons. Fire Imps follow the Caster AI.

They have a very small chance to drop a Flower of Fire.

A good strategy for dealing with multiple imps and not taking a lot of damage from their volleys is to use a fast weapon while rapidly hitting left and then right repeatedly. This seems to reset the swing animation allowing for a much higher chance to block the projectiles.