The Demon (and the Arch Demon on Old-gen console and 3DS) is an enemy found in The Underworld. It attacks by summoning volleys of four spinning purple blades that accelerate towards the player, which are similar those fired by the Demon Scythe spell that it has a chance to drop. These blades have a chance to inflict Darkness and are stopped by any block, but cannot be destroyed with weapons.
Like other Underworld enemies, Demons are immune to lava. A rarer variation of the Demon is the Voodoo Demon.
The Crystal Storm is an extremely effective weapon in pre-Hardmode, and even well into Hardmode. Obtaining it early from Arch Demons may be worth the lengthy farming.
In the real world, demons are supernatural beings prevalent historically in religion, occultism, literature, fiction, mythology, and folklore; as well as in media such as comics, video games, movies and television series.