Nightmareland is a dream dimension made up of an individual's worst dreams. It is ruled over by the Nightmare King and is where Creepy Eddie is from.
Nightmareland appears in the 1987 cartoon episode Nightmare in the Lair. When Donatello's invention, the Dream-O-Vision malfunctions, he instructs everyone to avoid using the machine. Michelangelo sneaks back to the lair to use the machine and winds up in Nightmareland. Leonardo goes looking for his errant brother and is also sucked into Nightmareland by the Dream-O-Vision.
Both Nightmareland and Creepy Eddie reappear in the Saturday Morning Adventures comic A Very Creepy Sequel. It is Hallowe'en night and the Turtles are on patrol when Eddie appears, gobbles them up, and deposits them in Nightmareland. Eddie shows them a yellow brick road and tells them if they reach the end of the road by midnight, he'll send them home. However, if they don't, then they are trapped in Nightmareland forever.