Metroid Prime

The Metroid Prime is a phazon-producing creature of unknown origins with genetic similarities to the species known as Metroids.

Origin
It is unknown where the Metroid Prime originally came from. What is known is that a Phazon meteor struck Tallon IV and in Chozo lore, it says that along with the Phazon came The Worm, supposedly the Metroid Prime. The Chozo sealed the Metroid Prime the crater from which it came, called the "Impact Crater." The Metroid Prime is believed to be one of the main causes of the Chozo extiction on Tallon IV, along with the Phazon it created.

Research by the Space Pirates
When the Space Pirates came to Tallon IV, they found phazon, and along with it, the Metroid Prime. Somehow, the Space Pirates managed to contain and study the Metroid Prime. They named it the Metroid prime due to its energy-absorbing abilities as well as because it shared genetic similarities to Metroids. Unfortunately, the study of the Metroid Prime was cut short by its virtual elimination of all research-division Space Pirates on Tallon IV, so further data on the makeup of the creature or its exoskeleton is limited.

Fight with Samus Aran
When Samus Aran engaged the Metroid Prime, she damaged it extensively, to the point where it was forced to shed its exoskeleton. What was revealed was a floating blue transparent membrane with a brain of sorts, a humanoid face, and multiple tentacles. This form of the Metroid Prime could spawn regular and mutated Metroids, and was impervious to all weapons save one: Samus's phazon suit arm-cannon supercharged with phazon. With extensive damage to its body, the Metroid Prime reverted to a much simpler body, the brain with a blue membrane and several small tentacles.

Death of the Metroid Prime
The core of the Metroid prime was weakened, and grabbed out at anything nearby. It grabbed onto Samus's suit, and began to pull her in. Samus fought against it, but the Metroid Prime somehow removed the Phazon suit, leaving Samus in her gravity suit. The Metroid Prime then started to expand and die, and while the impact crater collapsed, Samus escaped. (Metroid Prime (game))

Birth of Dark Samus
The Metroid Prime was not completely dead, however. Somehow, it possessed Samus's Phazon suit, effectively giving it a new body. This being eventually became known as Dark Samus. Dark Samus fed off of Phazon much like the Metroid Prime. When Dark Samus came across Aether, another planet hit by a Phazon meteor, she found phazon and began to feed. The first time she encountered Samus Aran again, she showed little resentment, only the desire to feed on Phazon. However, she did destroy a light crystle protecting Samus in the dark world, leaving her to die.

Space Pirate Base
The first few meetings between Samus and Dark Samus were by chance. Dark Samus was attempting to feed off of Phazon in a Space Pirate base located at Agon wastes. Samus, doing recon work in the Space Pirate base, encountered Dark Samus. They fought, and Samus was the victor. Dark Samus dematerialized into small, blue particles, but wasn't dead. She later encountered Samus in Torvus Bog, but didn't engage in combat for unknown reasons.

Sanctuary Fortress
Dark Samus went to the Sanctuary Fortress to feed on the Phazon there, and again encountered Samus Aran. By this time, Dark Samus was more powerful due to phazon consumption, and vengeful. She engaged Samus Aran on an elevator platform, and demonstrated new abilities such as an improved charge shot. Samus severely weakened and almost killed Dark Samus. Stumbling, Dark Samus fell off the elevator platform when it reached the top, and off of the Fortress before Samus could finish her off.

Final encounter
After Samus defeated the Phazon-mutated Emporer Ing, blue particles appeared where he used to be, suggesting Dark Samus was inside the Emporer Ing. Dark Aether began to collapse without any energy to sustain itself. When Samus left the Sky Temple and attempted to leave Dark Aether, she was confronted by Dark Samus. By this time, Dark Samus had absorbed so much Phazon that she was dangerously unstable. Dark Samus underwent some physical changes, including her armor becoming transparent, showing veins and a skull revealing three eyes. Dark Samus was invisible to Samus's Dark visor, one of the contributing factor's to her last defeat. Dark samus, due to her phazon mutation, grew more combat powers, such as the ability to create a phazon force-field around her. With this force-field, she was impervious to all weaponry except one- a phazon charge beam. Dark Samus' vented out Phazon particles in frequent intervals, and the Phazon particles she sent out were captured by Samus's Power Beam and used to damage Dark Samus. Dark Samus was weakend and fell to the floor. She slowly walked towards Samus, trying to touch her, but split into her particle form, which faded away. Samus then escaped and Dark Aether was destroyed.

Rebirth
Dark Samus was not destroyed, however. Above Aether, blue particles came together, forming Dark samus once more. (Metroid Prime 2: Echoes)