Comic book supervillain owned and published by Marvel Comics. He was created by writer Stan Lee and artist Steve Ditko, and he made his first appearance way back in Strange Tales #110 in July 1963, in the very first story featuring Doctor Strange. A godlike supernatural entity, he is the ruler of the Dream Dimension, and he is a member of Dr. Strange's rogues gallery, though he has bedeviled almost every major Marvel superhero at one time or another.
Nightmare is a tall man with angular features, pale skin, shaggy black or green hair, a green bodysuit (sometimes plain, sometimes patterned or textured) and a tattered cape. He looks, almost certainly by coincidence, a lot like Morpheus, Neil Gaiman's Dream King from the "Sandman" comics. He sometimes rides a horned black horse called Dreamstalker. Nightmare and humanity actually need each other to survive. Without humanity, there would be no dreams, and Nightmare would cease to exist -- and if Nightmare were killed permanently, humanity could no longer dream and would go insane.
Nightmare tends to show up as a behind-the-scenes schemer, trapping heroes in their own dreams and nightmares, or plotting to steal more psychic energy from people he torments in their sleep. He's teamed up with a variety of powerful demons to tackle Dr. Strange or Ghost Rider, but despite his near-omnipotence, he doesn't have a lot of power over anyone who's awake -- and if he manifests in the waking world, he could very easily get his butt whupped by any superhero who feels like doing it. Most of his plans revolve around manipulation, trickery, or trying to pull off some grand scheme before anyone else is aware of it, like when he tried to take over the entire concept of fiction, only to be foiled by the Fantastic Four...
He has at least three children -- the Dreamqueen, another dream scavenger with her own Dream Dimension; Daydream, a being who sustained herself benevolently on daydreams; and Trauma, a teenaged superhero named Terrence Ward, who was able to shapeshift into an opponent's fears. All three of these characters were conceived by rape -- in fact, Daydream's mother was Betty Ross Banner, the (at the time) recently deceased wife of the Hulk. This got Nightmare temporarily killed when the Hulk found out.
Nightmare has appeared in a number of animated cartoons. He was voiced by Jim Parsons in "The Super Hero Squad Show," where the heroes actually recruit Nightmare to their side, temporarily, by giving him a membership in the Cheese of the Month Club. He was voiced by Mark Hamill and Drake Bell in the more serious-minded "Ultimate Spider-Man" and by Matthew Waterson in an animated movie called "Hulk: Where Monsters Dwell."