Phaaze is a sentient planet that is the source of the chemical Phazon. It is the Bigger Bad of the Metroid Prime videogame tetralogy because it is where the creature Metroid Prime originates from. The Phaaze is an organism of gigantic dimensions, planetary dimensions to be precise, and it spawns Leviathans within its wombs every century.
Despite planet Phaaze being the Bigger Bad, Dark Samus was powerful enough to eventually take full control of it and direct all of its actions.
Speculation about a sentient planet was rumoured among the Chozo for years after their encounter with Phazon; as they wondered where the meteor originated from and where Phazon came to be. Their worst nightmare was that an entire planet could be coated with this toxic mutagen; and their fears would come true. The Leviathan which hit planet Tallon IV was from Phaaze, and Tallon IV was swiftly transforming into a clone of Phaaze during the time the Leviathan spread the chemical. The Chozo saw in visions Samus coming to destroy the Phazon and they left behind weapons for her, but they did not suspect that she would destroy the very source of Phazon itself.
A Chozo Searcher on the planet Elysia discovered data transmitting from a long-lost space probe, which transmitted information about a "sentient planet" which was apparently covered with Phazon. The Searcher did not learn much more because the encounter with Phaaze was brief, but the Chozo learned the entire planet was conscious and sentient. Data was lost and for centuries Phaaze lay undetected. However, Metroid Prime, reborn as Dark Samus, eventually brainwashed the Space Pirates for power and armies and led a squadron to Phaaze, to which planet she had returned. She had controlled all the Phazon on the planet and needed a device which could empower her to control the planet itself, so she made the Pirates raid a ship named the G.F.S. Valhalla which was on a scouting mission, and the Pirates stole the ship's sentient computer, the Aurora Unit 313, which Dark Samus took to Phaaze and connected the computer to the planet. Aurora Unit 313 transmitted data to the Galactic Federation before capture, and the Federation alerted Samus Aran. Samus killed the Leviathans on planets Norion, Bryyo, Elysia and the Pirate Homeworld; and then she and the Federation travelled to Phaaze via a Leviathan which they'd captured and bought under their control.
Within the planet Phaaze is a womb where creatures called Leviathans develop. Once they fully grow, they leave Phaaze and impact other planets, spreading Phazon, turning the planet into a clone of Phaaze within a few years.
As soon as Samus landed, the atmosphere of pure Phazon made her upchuck Phazon and become so corrupted that her gunship no longer recognized her. Samus then explored Phaaze and found many strange creatures such as Phaaze-Ing, relatives of the Ing which had corrupted and plagued planet Aether. She also found Aazelion, a strange beast which tried to rape her. Eventually, Samus jumped down a shaft after killing a Leviathan, and wound up in the heart of planet Phaaze. She battled Dark Samus, who merged with the Aurora Unit 313 and Samus defeated the computer. Eventually, Dark Samus exploded into oblivion, and then Phaaze began to become self-destructive due to an overload of Phazon. Samus escaped and flew out just after the Federation, at a time Phaaze violently exploded, ending all Phazon in the cosmos. She and the Federation then rejoiced at the destruction of all Phazon in the Universe.