Fire Imp

Fire Imps are a Caster enemy found only in the The Underworld. They shoot three volleys of Burning Spheres 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 are not immune to lava and can be killed by knocking them into it. They can also drown when caught underwater but will most likely teleport away before suffocating.

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. Fire Imps will, however, follow the player if he tries to flee (unless he teleports away with a Magic Mirror, reaches level 0 or is killed), so they need to be dealt with eventually.