The Nightmare King  

The main antagonist in the story is a person who was sealed away behind a large door for an extended period of time, until he was released by Nemo under Flip's guidance. He then kidnaps King Morpheus to lure Nemo and the Royal Scepter (which both he and his fellow Nightmare Land inhabitants feared) to his castle. Despite facing numerous challenges, Nemo ultimately manages to destroy the Nightmare King with the scepter, thereby ending the evil king's reign and also putting an end to Nemo's own nightmares.