The origins and history of the Nightmare King are unknown, but he is apparently the embodiment of nightmares, he rules over Ghost Cave, a dark cave which represents all the fear and darkness of the world's nightmares.
- The Nightmare King bares strong resemblance to Chernabog.
- He also slightly resembles Black Doom from Shadow the Hedgehog.
- The Nightmare King is similar to Lord Nightmare from Kirby's Adventure and Wizeman the Wicked from NiGHTS.
- His face slightly resembles the Hellhound from All Dogs Go to Heaven.