Nightmare

Nightmare is described as a "a bio-mechanical organism designed for military applications" that "possesses uncommon power and can manipulate gravity". Nightmare appears as a boss in both Metroid: Other M and Metroid Fusion, in which Samus has to defeat the creature while enduring its gravity manipulation abilities.

Biography
Chronologically, Samus first encountered Nightmare in Metroid: Other M while on her journey to Sector Zero on the Bottle Ship. The creature fights Samus twice; once when she tries to enter Sector Zero, and once when she tries to exit Sector Zero. When first defeated, Nightmare's face will tear off while colliding with the walls due to the malfunctioning of its Gravity Manipulator, until the beast finally makes a thud on the ground. When Samus returns to the area, Nightmare will struggle to get up while making screeching noises, and the duo do battle once more. After the second battle, Nightmare will explode into pieces, leaving only the head intact with the rest of its body parts scattered. Its corpse rests on the ground until the post-credits sequence.

In Metroid Fusion, Samus first encounters Nightmare when leaving Sector 5 for the first time, where she catches glimpses of Nightmare flying around behind the large windows (possibly its containment tank), causing the entire room to shake violently. It also appears when she is entering and leaving Sector 5 while obtaining the Power Bombs, though it is never mentioned by Adam after any of these encounters. After Samus obtains the Plasma Beam, Adam detects an explosion in Sector 5 and tells Samus to investigate. When she arrives, Samus finds most of the sector in ruins. Exploring further, she eventually finds the X-infected Nightmare in a uniquely mechanized room. It is interesting to note that the Nightmare appears damaged. The damage adds to its nightmarish appearance, making it look decomposed and rotting.

Nightmare
Nightmare floats in the middle of the room firing energy blasts at Samus, and will sometimes spin in one place, firing energy lasers, and may even try to run through the platform to ram Samus. Nightmare will manipulate the gravity and slow Samus down. This will allow it to attack Samus more easily. Eventually, Samus will freeze its Gravity Manipulator, causing it to momentarily shut down. Samus is able to utilize this period of normal gravity to fire at Nightmare's face with Super Missiles, otherwise the Missiles would collide with the ground. This will destroy the faceplate and Nightmare's gravity powers will go out of control, causing it to slam into the walls of the room twice before falling to the floor. Believing Nightmare to be dead, Samus ventures off to Sector Zero.

Nightmare Unmasked
As Samus leaves the area after inspecting Sector Zero, Nightmare's body suddenly rises up and attacks her in its unmasked form, having regained its health. Nightmare has the same attacks as before, but will now fire a laser from its face, as well as use its gravity manipulator to create large vortices that suck in all of Samus' beam and missile fire. Occasionally, the unmasked Nightmare will fall to the ground, only to rise up again. Nightmare is also much easier to deal with now that the Gravity Suit is activated and Samus can now fire her missiles even when the gravity is increased. When defeated, it will explode, leaving the arms and tail shattered into pieces and the destroyed body laying on the ground, officially defeated.

Metroid Fusion
In Metroid Fusion, Nightmare has three phases, not including its Core-X form. It remains mostly the same as in Metroid: Other M.



Phase 1
During this phase, Nightmare will stay in front of Samus and shoot bursts of lasers from its arms at regular intervals. These bursts can be difficult for Samus to jump over as it shoots them in a spread. Its weak point is the Gravity Manipulator - the glowing purple protrusion connected to its central body. Normally, its arms will be blocking it, so Samus must lure Nightmare up by jumping high, then coming down and getting a shot or two at it using the charged Plasma Beam or an Ice Missile. Once it has been sufficiently damaged, the battle enters its second phase.

Alternatively, Samus can stay under Nightmare in its first phase, as the creature only stays in one spot while hovering horizontally, and whenever Nightmare would come down, Samus could roll into a Morph Ball and stay out of harms way, then come back and shoot the manipulator with more missiles and charged shots.

Phase 2
Nightmare's Gravity Manipulator starts running, lowering the height of Samus' jumps and the distance which her missiles may travel, forcing Samus to rely on her charged Plasma beam. Nightmare will begin to move slowly back and forth; its eye sockets also begin to emit a slimy, green substance. After taking sufficient damage, the Gravity Manipulator is destroyed, returning gravity to normal. Nightmare's face mask then explodes, revealing its true face, upon which it enters its third and final phase.

Phase 3
Nightmare will begin to move slowly forward while firing its energy beams, and then fly erratically around the room, becoming difficult to dodge. Its exposed face functions as its weak spot. Samus must utilize the ladder and the Space Jump to effectively avoid attacks and aim counterattacks. When Nightmare (along with its Core-X) is defeated, Samus' Gravity Suit is restored.

Trivia

 * It is unknown when exactly Nightmare became an X-host, though it can be safely said that it was uninfected before Samus acquired the Ice Missiles, as it only began flying around erratically in Sector 5 after she obtained the concussive weapon (this would also explain the seemingly senseless destruction it caused in the sector afterwards).
 * Nightmare likely got its name from its mutant-like face, the phantom-like body structure, the intensive wailing, and that its abilities would be terrifying on the battlefield.
 * The boss is found inside a room in which the background is composed of scraps of metal, machinery and cables. Whether or not Nightmare caused this mess is unknown. Metroid Prime and Metroid Fusion: Prima's Official Strategy Guide calls the room a "junkyard".
 * There is an enemy in the Nintendo DS game, Mario & Luigi: Partners in Time called a "Snoozorb." It shares similarities in body plan, face, and name (pertaining to sleeping). Snoozorbs attack by either shooting a laser beam or changing into a ball shape and rolling into the Bros.
 * Unlike the Nightmare-X in Metroid Fusion, the original Nightmare's face in Metroid: Other M does not leak out of its mask, nor does its face melt when fired at the unmasked figure.
 * It would seem the X Parasites are capable of taking the shape of inorganic materials, as Nightmare's main metallic body is revealed to be part of the Core-X and regular X during its death animation in Fusion. Based on Samus' final battle with the SA-X, the X uses organic material and molds it in the shape of artificial components that the species can't directly infect/possess.


 * After defeated a second time in Metroid: Other M, Nightmare's destroyed body lays on the ground of the room it was fought in, until the post-credits sequence, at which point it is no longer there. This implies that the Galactic Federation moved the majority of its body to the Biologic Space Laboratories research station for its appearance in Metroid Fusion, along with Ridley.
 * The unmasked Nightmare in Metroid: Other M shares its theme with Brug Mass, as well as Fune and Namihe.