Leviathan (Metroid)

Leviathans are semi-sentient creatures that travel through space and corrupt planets by impacting them and spreading Phazon. They are born in a womb in the Genesis Chamber on the planet Phaaze. There have been five known cases of Leviathan impact, one captured by the Galactic Federation, and one infant in the Genesis Chamber.

Tallon IV
On the planet Tallon IV, the Chozo predicted the impact of a stellar object that would bring a "great poison". They also foretold that a "worm" would infest the planet and devour it from within. This prophecy eventually came to pass; A Leviathan eventually smashed into the planet, causing the spread of Phazon, either killing or corrupting all the Phazon came in contact with. The remaining Chozo eventually built a temple, called the "Cradle", to contain the Leviathan and the Phazon. Although the Chozo lived on as ghosts, the Phazon still affected them, turning them into vicious, irrational creatures that would attack anything on sight.

Aether
On Aether, a stellar object, revealed to be a Leviathan, struck the planet, tearing open the fabric of space, and splitting Aether into two dimensions; Aether and Dark Aether. The Ing, the species of Dark Aether, invaded Aether, where the Luminoth resided. Although the Ing were nearly capable of exterminating the Luminoth, Samus arrives and manages to restore the power to Aether and defeat the Ing Emperor.

Pirate Homeworld
Unlike the other cases of Leviathan impact, the Space Pirates wanted a Leviathan on their planet, knowing the military benefits and power of Phazon. As most were already brainwashed by Dark Samus and her followers, the pirates rejoiced that the "Dark Hunter" would bring a Leviathan to their world.

Biology
Leviathans vaguely represent the actual Leviathans of mythology, where they are shaped like squids. They have tentacles that propel them through space after being released from Phaaze. Each Leviathan has a core that relies on a phazon-corrupted creature for protection, which is usually a local bioform. It is revealed that there is a control center within the Leviathan when Samus infiltrates one that orbits the Pirate Homeworld. Leviathans are also known for thier ability to open wormholes, which allows them to travel great distances in short amounts of time.