Demon

The Demon (Official name is unknown.) is a new enemy type to come in the Terraria patch, 1.0.5, it is proven in Tiy's latest blog post on the Official Terraria forums. It seems to be a flying enemy (Attack unknown, might fire fireballs.) and will likely be in The Underworld biome.

Trivia

It is rumoured That the Demon will drop the new Demon Scythe spell book as seen in this video: http://www.youtube.com/watch?v=w1EgEnivvkY