After the defeat of King Dedede, Kirby put the Star Rod in the Fountain of Dreams unknowing of Nightmare. Nightmare gets freed from the Star Rod and clashes with Kirby. In this dimension Nightmare wins. Kirby has failed. Nightmares and darkness fall upon Dreamland leaving the residents in eternal fear and some madness.

you are being tormented in a violent nightmare

Hehehehahaha! Your hero has failed! Welcome to your new home! Endless nightmares and chaos! So exciting! My reign will replace those boring dreams with nightmares!