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, after they get hit, 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.

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

Fire Imps seem to require a target space of 4 block high by 3 blocks wide to teleport into a location.

Strategy
An interesting side effect of the hell mob code is that most of the time, two imps will block all other mobs from spawning. This translates into not having to deal with any bone serpents while you are mining hellstone, so long as you constantly deflect the fireballs. Assuming you have a cobalt shield, you can continuously mine with relatively little disruption from them.