He is entirely black and goopy skeleton, with 4 tentacles coming out of his back. His left eye is constantly illuminated in battle blue. When Nightmare was younger or not corrupted he and dream lived in a town under a tree, split between Night and Day. UnCorrupted Nightmare was actually called Night.
He's leader of The Bad sanses. Dust, Killer, Horror, Cross, Error. You're his child. His successor in being guardian of negativity. You're one of the strongest because you were born pure negative. 