Phaaze

Phaaze is a planet in the Metroid universe and the source of the mutagenic substance known as Phazon. However, it is not merely just a planet but a single colossal life-form. This sentient planet existed in the farthest reaches of the Milky Way galaxy and was impossible to reach by conventional FTL travel methods. Using a combination of Space Pirate technology and the wormhole-generating capabilities of the living planet's own offspring, the Galactic Federation and bounty hunter Samus Aran travelled to Phaaze in order to destroy the entire planet and thus put an end to the Phazon Crisis plaguing the galaxy.

Biology
Phaaze is comprised entirely of Phazon and supports a harsh environment that is deadly to any and all non-Phazon life-forms. Its biology is highly complex, but it is capable of producing and birthing offspring known as Leviathan Seeds within great cavernous wombs that run deep into its core. Every 100 years, Phaaze births a Leviathan which then journeys across the stars toward a life-supporting planet, impacting said planet and contaminating it with Phazon. Over the course of centuries, the affected planet will be completely corrupted by the Phazon and become a clone of Phaaze itself.

Discovery
Phaaze was first discovered by a Chozo Searcher on Elysia. A once-thought-lost probe began transmitting data on Phaaze. However, this data only revealed minor details about the planet. The Chozo Searcher learned that Phaaze was somehow "alive". Unfortunately, the data being collected was mysteriously lost before the Chozo could properly analyze it. Further research about the planet was next to impossible, as Phaaze was incredibly far away, even for Chozo space-travel standards.

Recent history
After this, Phaaze would be undetected for many years. The next time that bioforms exotic to Phaaze would lay eyes on it would be when Dark Samus brought the Space Pirates there. Following the Space Pirate attack on the G.F.S. Valhalla, Dark Samus implanted Aurora Unit 313 into the planet itself, allowing her to control the planet and its resources.

Finally, the Galactic Federation tracked the origin of the Leviathans to Phaaze. They began preparing to travel there to put an end to Phazon's destructive power. After taking control of a Leviathan Battleship circling the Pirate Homeworld and warping there, the Federation engaged the Space Pirate fleet in orbit above the planet while Samus headed down there to find Dark Samus.

After traveling through Phaaze's depths, Samus confronted Dark Samus. After they did battle, Dark Samus called forth Aurora Unit 313 and merged with it. Samus destroyed the corrupted Aurora Unit connected to Phaaze, and due to the symbiotic nature of this relationship, the destruction of the Aurora Unit caused a chain reaction that culminated in the destruction of Dark Samus and planet Phaaze itself.

Native life-forms
Phaaze itself is a life-form and also supports a wide variety of Phazon-based creatures both spawned within the planet itself or other creatures from different planets that have been corrupted. Only life-forms that have been positively identified are listed here.
 * Aazelion - Sedentary creature that tends to remain still until prey enters its personal space.
 * Jelsac - Crab-like creature that inflates its body and explodes, releasing toxic venom.
 * Leviathan Seed - Offspring of Phaaze resembling meteors. These creatures are sent out across the galaxy to corrupt other planets with Phazon.
 * Phazon Metroid - Metroids corrupted by Phazon that can become intangible. Metroids were originally engineered by the Chozo and are not native to Phaaze, so it is unknown how they came to inhabit this planet. It is suspected that the parasitic creatures were originally brought to Phaaze by the Space Pirates.
 * Hopping Metroid - Mutated Metroids that strongly resemble Hoppers.
 * Metroid Prime - No living Metroid Prime specimen has been encountered on Phaaze, but Samus Aran did witness the presence of several huge insectoid husks that resemble the exoskeletal form of the Metroid Prime encountered on Tallon IV. Given that Metroids are not native to Phaaze and the Space Pirates could not have brought the creatures to Phaaze until recent times, it may be that the so-called Metroid Prime is not a Metroid at all but a separate life-form with genetic similarities to Metroids. This is, of course, entirely speculative.
 * Phaz-Ing - Amorphous creatures that shift between fluid and solid form. Their resemblance to the Ing forms called Inglets is merely a coincidence.
 * Phazon Hopper - Corrupted Hoppers from Bryyo presumably brought to Phaaze by the Space Pirates.
 * Metroid Hatcher - Giant Phazon Metroid undergone further mutation. Spawns Phazon Metroids from its maw.
 * Phazon Puffer - Amorphous creature that expels toxic gas. Explodes violently when attacked.