Fire Imp

The Fire Imp is a mage found in The Underworld. They are by far the most annoying creatures in Terraria. Fire Imps will teleport around and shoot a spell called Burning Sphere (1HP, 25 attack power, 0 defense) at the player. Burning Spheres travel through walls and can be destroyed if at least 1 damage is applied against them. Fire Imps' attack pattern is identical to that of Dark Casters and Goblin Sorcerers. If a player touches a fire imp, they will take damage as if they were burned. The Obsidian skull does not affect this.

Imps drop coins and also the Flower of Fire as a rare drop, the Plumber's Hat, as well as the Whoopie Cushion.

They cannot spawn in tunnels three blocks high by two blocks wide. They can spawn on platforms but platforms do not count as a ceiling. If one tunnels deep enough with no open spaces around they will stop spawning altogether. This is an effective way to mine Hellstone ore very quickly at the bottom of the map.

Fire Imps shoot three burning spheres then pause for a second before teleporting and repeating, which continues until it dies or despawns. In addition, they will teleport early if they have been attacked. They will occasionally teleport on top of a player dealing damage. While being attacked or falling through lava, they are unable to teleport or shoot As of the 1.0.5 update, Fire Imps are immune to lava.

Fire Imps are annoying to deal with in the Underworld. It is recommended to bring a Cobalt Shield (removes knockback) and/or Obsidian Skull, and if not either, an Obsidian Skin Potion. Any of these help so that you can easily maneuver around without being hurt by Hellstone. A good weapon to fight Fire Imps in groups is the Vilethorn, since it goes through walls, constantly deals damage, and keeps them from teleporting or casting Burning Spheres.

Fire imps have an uncommon spawn rate and shoot slower as of the 1.0.5 update.

As of update 1.0.4, Burning Spheres no longer drop hearts or coins. Also, Burning Spheres can be blocked by using any flail besides the Harpoon by holding down the attacking mouse button. This has a barrier-like effect on the spheres, but the spheres can still hit you from some angles.

Trivia

 * Due to the Fire Imp's ability to teleport, a player can lure the Imp to the surface by using a Grappling Hook, or other device to quickly ascend while the Imp is following the player.