Leviathan

Leviathans are a crucial plot element of Metroid Prime 3: Corruption, spreading Phazon across various planets in the story. Their sole purpose is to spread Phazon from the planet Phaaze to corrupt planets across the galaxy in order to create more Phazon-based planets, similar to Phaaze. The core of the Leviathan is very strong and can only be destroyed by a stream of Phazon or a powerful laser blast. These creatures were notable not only for their ability to create Phazon, but mostly because of their most powerful ability, which allowed them to appear from nowhere in seconds: wormhole generation.

Infancy
Leviathans are the "children" of Phaaze. Born in a serpentine organ deep within the planet, they feed on vast amounts of Phazon energy until they develop a Phazon core, which marks their transition into adolescence. Before this, they are extremely unstable, though naturally extremely resistant, and cause massive explosions if damaged before full maturity. Once they reach adolescence, the Leviathans are slowly pushed upward to the surface until they mature to their adult size. The oldest Leviathans are up closest to the surface while the youngest are below. As Leviathans are launched into space, the younger ones move closer and closer to the surface. Once Phaaze has found a planet to corrupt, it launches the Leviathan into space, in hopes of impacting a planet. However unconfirmed this is likely the creature on Metroid Prime 3: Corruption's title screen.

Actions
After ejecting from Phaaze, the Leviathan will create a wormhole to shorten its journey to the intended planet. The Leviathan will home onto its closest target and impact the planet, spreading Phazon from the collision. Eventually, the Phazon core of the meteor slowly seeps its way into the planet, spreading Phazon and replacing the ecosystem with one that is Phazon based. The Leviathan attracts a native creature and corrupts it with massive amounts of Phazon. The corrupted creature serves as the Leviathan's guardian, to protect its Phazon core. Sometime after this, the Leviathan itself will die, leaving behind the "husk" of it's body.

Controlled by Dark Samus
Once Dark Samus returned to Phaaze, with the help of the stolen Aurora Unit 313, she was able to control when and where a Leviathan would launch and land by fusing them with the stolen technology. This allowed her to spread Phazon throughout the universe far more effectively and strategically. She used this ability to launch four Leviathans: one to the Pirate Homeworld (to corrupt the resident Pirate forces and thus secure a powerful army), one to Bryyo (to seize control of the planet's priceless Fuel Gel and cut off the Federation's supply of it), another to Elysia (a Federation spy base, originally built by the Chozo), and the fourth to Norion (one of the Federation's greatest strongholds).

While the Leviathan aimed for Norion was destroyed before impact, the other three thrived and were able to pour great amounts of Phazon into each planet. The ones in Bryyo and Elysia were protected by great energy shields, while the Pirate Homeworld one was so heavily defended only one cargo route was allowed to reach the Leviathan itself. In orbit around the Pirate Homeworld, the Space Pirates had outfitted a fifth Leviathan with weapons; however, with Samus's help, the Federation took control of it, using its wormhole-generating abilities to warp to Phaaze.

Impacted Planets
Fifty years prior to the first Metroid Prime, a Leviathan impacted Tallon IV and brought Phazon to the planet. There was a creature living within this Leviathan since its launch from Phaaze, named by the Space Pirates as the Metroid Prime, that was feeding on the Seed's Phazon and might have been the Leviathan's guardian. Several days after its impact, the Chozo residents of Tallon IV were able to greatly slow down, if not completely stop the spread of Phazon on the planet by building a temple named the Cradle, which used an unseen containment field. This particular Leviathan seems to have many characteristics that differ from its brethren, such as having a higher lethality rate towards local populations than seen on the Pirate Homeworld. This suggests that the molecular composition of the Phazon brought to Tallon IV, and possibly also that which was brought to Aether, were from fully matured Leviathans compared to the ones seen in Metroid Prime 3: Corruption, as Dark Samus (a.k.a, the Metroid Prime) may have been impatient to wait for them to grow and thus launched them prematurally. Since there is no visible sign of the Leviathan itself in the game, it can be assumed that the creature died once the Chozo erected their temple around it. This might explain why the hypertoxic substance found in the impact crater was not even seen on Phaaze: it was probably produced by the decay of the Leviathan's body or the surrounding environment (perhaps the magma close to the planet's core had changed its composition and color).

Twenty years before that, a Leviathan impacted planet Aether, visited in Metroid Prime 2: Echoes. However, due to the instability of Aether's planetary energy, another dimension, known as Dark Aether, was ripped open; the Leviathan itself, along with nearly all of its Phazon, were teleported to Dark Aether at the moment of impact. It is speculated that the Phaz-Ing, transported from Phaaze and living inside the Leviathan, were altered on a subatomic level by Dark Aether's atmosphere to become beings of darkness called the Ing, and that the Emperor Ing was the corrupted guardian. However, it is also possible that neither the Seed nor its guardian were encountered in the events of Echoes and that both were annihilated indirectly when Dark Aether collapsed.

Several months after the events of Metroid Prime 2: Echoes, a Leviathan impacted the Space Pirate Homeworld. This was the first one to be controlled by Dark Samus. Ridley was chosen to be its guardian.

A few months after that, Dark Samus sent three more Leviathans to corrupt a number of planets. They were headed to Norion, Bryyo, and Elysia. The one aimed for Norion was destroyed before impact, but the ones headed to Bryyo and Elysia were unharmed until Samus Aran destroyed the Phazon cores within them. The Bryyo Leviathan's guardian was a corrupted Mogenar war golem (the place in the game Samus first encounters a Leviathan, after she defeats the Mogenar), while the Elysia Leviathan's was a strange robot called Helios. Interesting to note is the fact that the first Leviathan seen in Metroid Prime 3: Corruption was easily destroyed by Federation tech, while Luminoth and possibly Chozo technology, were unable to destroy their respective Leviathans at all. This is considered strange as both mentioned species are highly evolved sentient creatures whose technologies far surpass the Federation's. This leads to the mentioned theory that the Leviathans launched prior to Dark Samus' rule were at the peak of their growth, possibly having a stronger surface. Though in case of the Chozo on Tallon IV, it may have simply be due to the fact that the colony of that planet chose to build a colony from natural materials and so lacked the tech needed to destroy the Leviathan.

It is unknown if any other planets were corrupted by Leviathans before the events of the Metroid Prime series, though it is likely. If so, it is unknown if these planets "survived" the destruction of Phaaze, though it is possible since they would have been seperate "organisms".