The Nightmare King

The Nightmare King was a powerful demon, being the incarnate of nightmare itself. Much like the The Emperor of Night and Chernabog, he was an evil force of nature with no sense of good whatsoever. He met his end at the hands of Chernabog and the Firebird.

Dealings with Rasputin
During this war, the Nightmare King made an alliance with the Emperor of the Night, their main goal in the war to defeat Chernabog. The Nightmare King was slightly skeptical about their goal, but the Emperor of the Night remained determined.

During a meeting between the two, Rasputin came to the Emperor, pledging his allegiance in exchange for his help in the war. The Nightmare King was suspicious about the Russian sorcerer, but both he and the Emperor put their trust in him when Rasputin saved the Emperor's life from the centaur Nessus, who had been sent by Hades and Eris to kill him.

Fight With Chernabog and Death
Later, Hexxus, seeking to eliminate two deadly threats from the war, told the Nightmare King about a weakness in Bald Mountain's defenses which would possibly allow him to defeat Chernabog. However, the Spirit of Destruction also warned the Devil about the Nightmare King's plans, therefore pitting the two rivals in a scenario where only one of them could survive.

Chernabog sent Willie the Giant, the Ice Titan, and the Lava Titan to kill the Nightmare King, but he quickly dispatched them, proceeding to Bald Mountain. At first, the Nightmare King had the upper hand, easily defeating the Satanic Lord's minions and deflecting his attacks. However, victory would not be his. Chernabog summoned the Firebird, and the two forces of nature combined their powers to incinerate the Nightmare King, finally killing the Demon.