Metroid Prime

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

Origin
It is unknown where Metroid Prime originally came from. What is known is that a Phazon meteor struck the planet Tallon IV and in Chozo lore, it says that along with the Phazon came The Worm, likely what the Space Pirates refer to as "Metroid Prime". The Chozo sealed Metroid Prime inside the impact crater from which it came, and it has remained hidden there for many years. 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, Metroid Prime. Somehow, the Space Pirates managed to contain and study Metroid Prime. They named it Metroid Prime due to its energy-absorbing abilities as well as because it shared genetic similarities to Metroids. Unfortunately, the study of 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 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 human-like face, and multiple tentacles. This "core" form of Metroid Prime could spawn regular and mutated Metroids, and was impervious to all weapons save one: Samus' Phazon Suit arm-cannon supercharged with phazon.

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

Dark Samus
Metroid Prime was not completely dead, however. Somehow, she possessed Samus's Phazon suit, effectively giving it a new body. This being eventually became known as Dark Samus. Unlike Metroid Prime, which could create phazon, Dark Samus needed to absorb phazon in order to stay alive. 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 crystal protecting Samus in the dark world, leaving her to die.

Space Pirate Base
The first few encounters 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 severely angry at Samus. 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 Emperor 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)

Metroid Prime
Metroid Prime was a major source of pollution and suffering on Tallon IV, as it was able to produce Phazon, which mutated the wildlife and poisoned the environment. Space Pirate log entries detail the evolution of Metroid Prime via phazon exposure from a weaker creature to its arachnid-like form, which exhibits armored plating and mechanical outgrowths capable of generating defensive screens. The creature could shift these screens to protect itself against new weaponry. However, through a genetic flaw, it was vulnerable to one energy type at a time. This flaw is believed to be rooted in the fact that Metroid Prime's weapons are based on four elements including power, wave, ice, and plasma, and cannot fire them through a screen that disallows penetraton by that beam type. Metroid Prime, according to log entries, is capable of assimilating mechanical weaponry into its body — weapons presumably collected during the creature's encounter with the Space Pirates.

Metroid Prime essence
Metroid Prime without its exoskeleton was still very powerful, and was impervious to all weaponry except a phazon charge shot. With its tentacles, it could create a circular field of fire that would expand outward, damaging anything nearby. It could produce pools of phazon to damage nearby creatures, and dissapear from visual, thermal, or x-ray fields of vision. Somehow, it gained the ability to create Metroids, as well as Phazon variations of them. When Metroid Prime was defeated, it attempted to grab at anything in the area, including Samus, presumably in a desperate attempt absorb Phazon to sustain itself, and took her Phazon suit in the progress. Somehow, out of the pool of Phazon it degenerated into, it turned into Dark Samus.

Dark Samus
Dark Samus seems to be on an everlasting search for Phazon. It is unknown why she needs Phazon, although it is possible she needs it to sustain herself. Dark Samus herself is pure Phazon and Samus Aran's DNA, along with the suit taken from Samus. Dark Samus has the ability to turn into blue particles, presumably of Phazon. It has been shown that too much Phazon absorption results in instability, in which she needs to vent Phazon. It is presently unknown how to kill Dark Samus, or if she can be killed at all.