Nightmare is a bio-mechanical creation kept aboard the B.S.L. research station. It was designed for military applications armed with laser beams and the ability to manipulate gravity with a generator on its underside. It goes berserk after the X infest it, and begins tearing apart Sector 5 (ARC). Samus tracks it down and destroys it, recovering the Gravity Suit.
While fighting Nightmare it will increase gravity to make everything extremely heavy. Any missiles fired while its generator is active will fly into the ground. Samus's jumping ability will also be reduced, making some of the fight feel as though it is underwater. After Samus destroys the generator on its underbelly, it loses this ability, and leaves its face open to attack. As Nightmare takes more damage, its face will seemingly "melt" until nearly all of it is falling off its body.
Metroid: Other M
Nightmare appears as a boss in Metroid: Other M that Samus must fight twice on the Bottle Ship. First on her way to Sector Zero and second on the way back. In the Metroid storyline this would technically be its first appearance, but the second game it has appeared in. It has its trademark gravity manipulating powers but is much faster and more active than it was in Metroid Fusion. And instead of Samus destroying the generator, it can only be temporarily deactivated by freezing it over with the ice beam. Eventually the ice will melt and the generator will come back online, so the process must be repeated several times in order to defeat Nightmare.
During the first encounter with Nightmare it can fire six concentrated lasers from its arms to sweep around the room and launch energy spheres that home in on Samus' location. While its gravity generator is active, Missiles and Charge Shots will be sent off course and away from the weak point on its face. Occasionally it will leap onto the platform Samus occupies and dive at her with abandon.
Once Nightmare is beaten, its mask will break to reveal its face before seemingly dying.
The next time Samus has to travel through Nightmare's room to continue the story, it will spring back to life and attack again. It can now use a new attack to launch a small gravity well to a part of the room that will quickly swallow up any missile or charge shot. Laser attacks are now fired through its mouth and it can still recklessly charge at Samus if its close enough. When it is defeated this time, its body will explode into pieces and will remain where it falls until the Galactic Federation recovers it at the end of the game, taking it to the B.S.L. Station.