Ruler of his own Nightmare World within the Dream Dimension (linked to and shaped by humanity's collective unconscious), Nightmare is a Class Three demon who influences the dreams of living beings as they sleep, feeding on humanity's psychic energies in particular. Nightmare occasionally singles out souls for special tortures, sometimes trapping their dream-selves in his realm. He has even sought to absorb the entire waking world into his own realm, though never with lasting success. Humanity's chief defender against Nightmare is the sorcerer Doctor Strange, who has fought the demon so often that Strange routinely casts a protective spell on himself before sleeping.
Nightmare has also faced many other foes such as Clea, Spider-Man, the Micronauts, Dazzler, Ghost Rider (John Blaze and Dan Ketch), the Hulk, Cloak & Dagger, Excalibur, Daredevil, Straw Man, Doctor Doom, Wolverine, Sleepwalker, the Enchantress, Heimdall, Morbius, Emma Frost, Captain America, Black Panther and Moon Knight. Nightmare has worked to corrupt unstable super-beings such as the Hulk and Cloak, each of whom have gone on mad Nightmare-inspired rampages, but his influence usually proves temporary. For a time, Nightmare was allied with rival fear-sowing entities such as the Fear Lords, but the group was thwarted by Dr. Strange and his allies.
Eight hundred years ago, Nightmare impregnated the demon succubus Zhilla Char; she died giving birth to their daughter, the Dreamqueen, who now rules her own nightmarish pocket dimension. For a time, Nightmare courted horror movie actress Roxanne in the mortal guise of Edvard Haberdash, helping her longtime director Lucre establish Club Fear where patrons paid to be terrified. "Edvard" eventually caused Lucre's demise, and Roxanne forced herself to keep dating the abusive, possessive, inhuman "Edvard" since she feared for her own safety and that of the world in the face of a wrathful Nightmare's power. During a recent terror attack, Nightmare exploited humanity's desire that it was just a dream, gaining a foothold on an earthly island where he can manipulate reality at whim. On Nightmare Island, the Hulk encountered and named Daydream, a daughter Nightmare had sired with a mortal woman in a deep sleep, who turned against her father and chose to dwell amongst humanity. Nightmare's natural enemies include the denizen Dusk of the hostile adjacent Realm of Madness, as well as the Gulgol, a monster who never sleeps.