Leviathan

Leviathans are a crucial plot element of Metroid Prime 3: Corruption, spreading Phazon across various planets in the story. They are massive, powerful creatures similar to an immense gasteropoid capable of stellar travel, protected by a thick shell that prevents any kind of damage. A Leviathan can be classified as a living creature, as it is born, grows up, and dies. However, its 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. 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.

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. Shortly after impact, the Leviathan dies; however, its gigantic shell will remain. Before its death, though, 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. Eventually, the Phazon core of the Leviathan slowly seeps its way into the planet, spreading Phazon and replacing the ecosystem with one that is Phazon based.

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 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.

Other Incidents
It is higly belived that in the first Metroid Prime, a Leviathan impacted on Tallon IV and brought Phazon to the planet. The Metroid Prime, therefore, was its Phazon-filled guardian.

In Metroid Prime 2: Echoes, the "phazon meteorite", that impacted Aether, could have been a Leviathan. However, due to the instability of Aether's planetary energy, another dimension, known as Dark Aether, was ripped open.