The Nightmare King is a powerful demon, being the incarnate of nightmare itself. Much like the the Emperor of Night and Chernabog, he is an evil force of nature with no sense of good whatsoever. He is the main villain of the animated film, Little Nemo: Adventures in Slumberland.
Disney Vs Non Disney Villains
Pre War: The Demon Lords
The Nightmare King appears in the time before the great war, as one of the three demon lords, along with Chernabog and the Emperor of Night. Having created a powerful crystal, in which all that was good would be destroyed and all villains would rise to power. Chernabog quickly decided to take all the power for himself, much to the King's fury. However, when the crystal's power was unleashed, the Dark Council intervened, and destroyed the crystal.
Revenge and Defeat
Seeking revenge for the doublecross, the Nightmare King set his sights on his brother, Chernabog. and attacked Bald Mountain. As both great demons sent their legions of spirits and demons into battle, all were sweapt away in the heat of the fury. As The King readied for a final assault, Chernabog summoned a great surge of demonic power, and detsroyed the King once and for all.
Non Disney Villains Tournament
Circus of Fear
The Nightmare King approaches Professor Screweyes, taking him on as a disciple. He has his minion set up a circus of fear which he uses to draw souls. The business is lucrative, but a significant interruption emerges when Samhain attacks. When Screweyes struggles to defeat the pumpkin ghost, the Nightmare King decides to take on the foe himself. Samhain sics several lesser demons against the Nightmare King, but the King devours them. Samhain tries to flee, only to meet the same fate as his minions.
Terminating a Traitor
Synonomess Botch, a disciple of the Nightmare King, decides to turn traitor and blow the Nightmare King up. The Nightmare King gets word of this and sends some demons to kill Botch. They fail, but they distract Botch long enough for the Nightmare King to creep into the scientist's lair and kill him.
Terminating a Contract
It becomes apparent that Botch's defection was part of a scheme by Valmont and Vlad Masters to steal valuable artifacts from Screweyes's circus. One of these artifacts empowers Masters to become his ghost form, Vlad Plasmius. Plasmius turns up the fear of the circus, compelling the Nightmare King to kill his failed disciple, Screweyes.