Leviathan (Metroid)

The Leviathan (also referred to as Leviathan Seed) is a colossal alien creature that appears in Metroid Prime 3: Corruption. Described as living meteors, Leviathans are the offspring of the living planet of Phaaze, the source of the mutagenic substance known as Phazon. The sole reason for their existence is to spread Phazon corruption throughout the universe. Once bred within the heart of Phaaze itself, they are launched across the stars towards life-sustaining planets, impacting the surface and spreading Phazon throughout the planet's environment.

Infant
Leviathans are bred asexually within a serpentine womb that runs through the body of Phaaze. While in development, they appear as wholly organic creatures, featuring transparent skin and clearly visible organs within the body. Bone, or chitin, formations are also visible, as well as a set of feelers on the bottom of the body and tendrils at the top.

Whilst developing in Phaaze's womb, infant Leviathans constantly feed on Phazon that permeates the womb. After enough feeding, they will develop a Phazon Core and produce their own supply of Phazon. The body will continue to grow until it has developed a hard outer shell of Phazite. After roughly 100 years of development, the Leviathan will be birthed from Phaaze, launched in space in the hopes of striking a habitable planet.

Adult
Adult Leviathans are covered with large patches of Phazon and have long tentacles growing out of their sides. The inside of a Leviathan appears similar to a cave containing various Phazon-based life and Phazon-generating organs. The passages that lead to the core chamber of the Leviathan appear as transparent tubes, similar to veins, but without liquid within them. There are doors in the Leviathan that seem to be a type of sphincter valve with a weak energy field covering it. The core chamber is a large spherical room, with many pulsating and moving objects which may be organs. The Phazon Core itself resides at the top of the chamber in a sealed sac-like sheath. The Core's appearance is that of a large, round, insectoid creature connected to the ceiling by several cords that it moves around with. It is covered in Phazon growths, has several eyes and a mandible-like mouth. The chamber is also usually where the Leviathan's guardian resides. Because the Leviathans on Tallon IV and Aether were not expressly shown, it is difficult to say which attributes of the three featured in Corruption are characteristics of immature Leviathans and which ones apply to all. After ejecting from Phaaze, the Leviathan will create a wormhole to shorten its journey to an intended planet. The Leviathan will home onto its closest target and impact the planet, spreading Phazon from the collision. The strength of this collision has been known to vary greatly, sometimes leaving huge scars upon the surface of the impacted planet. After collision, the Leviathan's Phazon Core, which it developed at the end of its infant stage, slowly seeps its way into the planet, spreading corruption and replacing the ecosystem with one that is Phazon-based. At this point, the Leviathan may possibly attract a native creature and corrupt it with massive amounts of Phazon. The corrupted creature serves as the Leviathan's guardian, to protect its Phazon Core. Some time after this, the Leviathan itself will die, leaving behind the "husk" of its body which will serve as armour to protect the core's environment against attacks from the outside.