Nightmare is the evil ruler of the "Dream Dimension", where tormented humans are brought during their sleep. He roams this realm on his demonic black horned horse, Dreamstalker. In physical form, he appears as a chalk-white man with green hair, a green suit, and a ragged cape. He was the first foe met by
Doctor Strange when a man who was having troubling dreams went to Strange for help, though it turned out to be repressed guilt for an act of murder. Nightmare then traps Strange in his realm and tries to feed on his power. However, Strange escapes the Dream Dimension after tricking Nightmare with an illusion of his enemies. Nightmare is a necessary part of human existence; without him to regulate fear through dreaming, humans would slowly be driven insane. At one point, Strange is forced to aid his enemy to prevent the latter's death from an inability to create new dreams. Nightmare also serves under
Shuma-Gorath and warned Strange that the demon would be a force that even the Sorcerer Supreme would have trouble defeating, and he once joined the
Fear Lords, a group of supernatural creatures who fed on fear, to attack Strange together. Their plans are undone when
D'Spayre tricks him into competing with the
Dweller-in-Darkness over who could frighten humanity more; recognizing that this would only allow D'Spayre to surpass them both in power, the two entities call a truce. Nightmare is the father of
Dreamqueen, a similar being who rules her own "dream dimension". She was conceived when Nightmare impregnated a succubus named Zhilla Char. Nightmare's realm is not part of the Mindscape, but the
Sleepwalkers are aware of him and consider him an enemy. Because Sleepwalkers do not have to sleep, Nightmare is unable to affect them. He seeks to do so by tormenting Rick Sheridan, the host of an individual Sleepwalker who had been trapped in his mind. Nightmare attempts to convince Sleepwalker that Rick is safe, but he is not fooled and sacrifices his chance to return to the Mindscape to stop Nightmare. In the
Tempest Fugit storyline of
The Incredible Hulk, it is revealed that Nightmare has been plaguing the
Hulk for years with hallucinations, misdirections, and manipulations of reality, by empowering himself. His second, more benevolent, daughter Daydream is also introduced in this story arc. Nightmare claimed that this daughter was conceived by forcibly entering the mind of the Hulk's late wife,
Betty Ross and raping her in her sleep. The Hulk kills Nightmare in retaliation, but he later returns to life. In "
Dark Reign", it was revealed that
Trauma is Nightmare's son and inherited part of his power over fear, being able to transform into the worst fears of those near him. Nightmare later manifests on Earth and battles the Avengers Resistance and the
Initiative. During the
Chaos War storyline, Amatsu-Mikaboshi (now adopting the title of Chaos King) has amassed an army of alien slave gods and is attempting to destroy the universe. He travels to Nightmare's realm while they are trying to torment Hercules with visions of Amatsu-Mikaboshi and quickly defeats the demon. Nightmare attempts to join Amatsu-Mikaboshi's forces, but he is not tricked by his begging and kills him. When
Serpent, the Asgardian god of fear, is released from his prison and launched the Serpent War,
Mephisto goes to the Infinite Embassy to learn what the other gods plan on doing about the Serpent, he visits several gods and locations, his last stop being Nightmare's chamber. Still unclear how Nightmare survived his encounter with Amatsu-Mikaboshi, Nightmare explains to Mephisto that the Serpent is starving him by taking all the fear. Nightmare wants to fight the Serpent, but Mephisto talks him out of it, citing that he will lose all respect from the other evil gods. Then, when Nightmare talks of joining the Serpent, Mephisto argues against it, claiming that the Serpent will keep starving him and he will die. Nightmare, unsure of what to do, listens to Mephisto when he advises for him to do what everyone else is doing: stay neutral, and wait for something else to happen. Nightmare is later seen tormenting
Loki as part of a plan to garner greater powers in
The Terrorism Myth storyline. Loki challenges Nightmare to a duel to the death, with the loser giving up their immortality. Nightmare accepts and is killed by Loki in the ensuing battle. Following the death of Doctor Strange, several mystical threats are able to freely move into position to try and doom the world of man, including Nightmare, who quickly finds himself in New York where he confronts
Cyclops,
Jean Grey and
X-23 during their sleep. However, Jean manages to break free from the nightmares and confront Nightmare on equal footing. ==Powers and abilities==