Lord Nightmare

Nightmare hails from both Kirby's Aventure and Kirby: Nightmare in Dreamland and is also seen in the Kirby cartoon, Kirby: Right Back at 'Ya.

The game
In Nightmare in Dreamland, Nightmare was torturing the residents of Dream Land by giving them nightmares. King Dedede was well aware of this and went to the Fountain of Dreams where Nightmare was, stole the Star Rod thus sealing Nightmare inside it until Kirby placed the Star Rod back to its rightful place, and then broke the Star Rod into to fragments to give to his friends to guard. Kirby was not aware of what lurked inside the rod until he put in back into place. Once Nightmare was free, King Dedede then swallowed Kirby and the Star Rod and spat them both out in Nightmare's direction. Nightmare had not been at full capacity when Kirby first fought him. In fact when the first Kirby/Nightmare battle commenced, Nightmare was only a ball of dark energy. When Nightmare was defeated the first time, he became his real form and fought Kirby on another planet that resembled the moon. Kirby was following this new form of Nightmare on Warpstar when Nightmare tried to take him down. One of Nightmare's star shaped shots finally connected and Kirby flew off of Warpstar and onto the battleground of which would be Kirby and Nightmare's final battle. Kirby's only weapon against Nightmare was the Star Rod itself.

The Anime
In the anime Kirby: Right Back at 'Ya, Nightmare runs a business called Nightmare Enterprises or N.M.E., a business that specializes in selling monsters to its clients. King Dedede happens to be one of his clients (can anyone say, irony?). The king is always ordering new monsters to get rid of Kirby and every one of those monsters ends up failing. The king never has the money to pay for any of these monsters and ends up eventually building up a massive debt that Nightmare keeps reminding of.

In the short movie, Fright to the Finish, Nightmare himself, the head of N.M.E., is fought and defeated by Kirby after Meta Knight leads an attack on the N.M.E. base.