Demon

Demons are found in The Underworld. In addition to causing physical damage if contacted, they shoot accelerating purple blades at the player, similar to the Demon Scythe spell, in volleys of four. These blades are stopped by barriers such as walls and ash blocks, and have a chance of blinding the player. The blades cannot be destroyed if they are hit by a weapon, unlike the fireballs of the Fire Imp which can be destroyed.

Demons that come in contact with lava will not get hurt (like most underworld mobs). A rarer variation of the Demon is the Voodoo Demon.

Demons are immune to the On Fire! and Cursed Inferno debuffs.

Trivia

 * The Demon's Demon Scythe attack is the only Magic attack produced by an NPC that can hit players through one-block thick walls.
 * The Demon resembles Firebrand, a Ghosts 'n Goblins enemy who starred in his own series Gargoyle's Quest. His most recent appearance was in Ultimate Marvel vs. Capcom 3.
 * In the console and the mobile version of Terraria, there is also a variation of Demons, Arch Demon. It is a stronger demon compared to its PC counterpart whose drops are the same as a PC demon. Its color is different but due its nature of being in hell it is difficult to distinguish its color. It seems to fire more Demon Scythes than the PC demon.
 * The Demon resembles the attack of the Tiny Devil from Castlevania: Aria of Sorrow

History
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