The Nightmare King is a powerful demon of darkness, the dark spawn lord of shadow and the destroyer of good dreams, one of the original Dark Spawn Lords during the first Cartoonian War, and is the main villain from Spongebob meets Little Nemo in Slumberland. He was sealed behind a large door long ago, until Nemo, under Flip's misguided influence, accidentally releases him. He kidnaps King Morpheus in order to lure Nemo, and more importantly the Royal Scepter (which he and the other inhabitants of Nightmare Land fear), to his Nightmare Castle. Nemo barely manages to destroy the Nightmare King with the scepter and ends the evil king's reign along with Nemo's own nightmares. It is hinted that the Nightmare King is responsible for Nemo's constant nightmares.
Role in the series
The Nightmare King reprises his role as stated above, but has an army of Villain leaguers working for him. It is later revealed that the Nightmare King is the true master of darkness in the Realm of Sleep, being the head Dream Eater Nightmare who created the Dream Eaters out of dreams of real-life animals and made them cute and cuddly-looking to trick unsuspecting mortals into a false sense of security so that the Nightmares can suprise their victims and eat their dreams. The Nightmare King is also revealed to be the creator of Anti-Black Coat, having created the silent warrior from Vanitas's dreams to prevent Sora from waking up from his eternal sleep caused by Young Xehanort, who inherited the power to control and create the Dream Eaters from the Nightmare King. Eventally, the full origins of the Nightmare King will be discovered from his former form, The Nightmare Jester.