Phazon Core (creature)

A Phazon Core is found in the center of all Leviathans. Samus Aran encounters these beings in Metroid Prime 3: Corruption after defeating the core guardian of the seed. Before the core reveals itself, Samus must use the power-up received after the battle (Hyper Ball, Hyper Missile, then Hyper Grapple as the game processed) to destroy certain objects around the room, which act as locks to the core. Destroying them causes Phazon streams to gush into a small opening in the ceiling and summon the Phazon Core. Samus must vent the extra Phazon out of her suit to overload the Core and destroy it. As Samus leaves, the core falls to the ground and explodes, sending Phazon flying in all directions. As Samus's ship flies away the Leviathan can be seen exploding and collapsing (Metroid Prime Trilogy and PAL versions only), bringing a stop to the spread of Phazon on that planet.

Biology
Leviathans are grown in serpentine wombs, deep within the planet Phaaze. While inside, they slowly reach maturity, living on a diet of concentrated Phazon fed to them through the womb from Phaaze. As this process happens, the Leviathan and its womb are pushed closer to the surface of Phaaze. Over time, Leviathans develop a Phazon Core, which allows the Leviathan to produce Phazon on its own. When the Leviathan is fully matured, it reaches the planet's overworld and is launched into space to corrupt other planets. Upon impact, the Leviathan attracts a powerful native life-form to enter it. The creature is then corrupted by large amounts of Phazon, and now serves to protect the Core of the Leviathan with its life. The Core then acts somewhat like Phaaze's womb; it begins feeding the corrupted creature with Phazon. In the case of the Bryyonian Seed, the creature that protects the Core, Mogenar, is fed with a cascade of liquid Phazon from the Core.

The Phazon Core resides in a chamber at the heart of the Leviathan, and is enveloped in a layer of protective skin while inactive. When provoked, the Core will emerge from the ceiling to assess the situation. The Core is connected to the Leviathan by a large tube sprouting from the base of the creature, which leads under, behind and above the core and which also splits into several smaller veins that suspend it from the ceiling. The Core itself a grey mass that is insectoid in appearance. Its method of vision is a singular blue eye. Beneath the eye is a set of two primary mandibles and four secondary ones underneath them. Two feelers emerge from either side of the Core's eye, and blotches of Phazon can be seen dotted around the Core's body. Finally, two antennae stem from the top of the Core, which lead into another set of veins to the ceiling.

Metroid Prime Trilogy art booklet
"When a Leviathan strikes a planet, it creates a Phazon Core which emits a strong toxin called Phazon all over the planet. Phazon is a powerful substance with the ability to completely corrupt living beings through genetic mutation. Over the course of a couple years, Phazon corrupts the entire planet, at which point the planet is reborn as a Phaaze clone."

Trivia

 * In Metroid Prime, even though Samus goes to the room in the Impact Crater that would hold the core, it is missing; in the place where the Core would normally be connected to, Metroid Prime is instead suspended. This lead to a theory that Metroid Prime completely absorbed it through its unsatiable hunger (a trait found in all Metroids) for Phazon, granting the creature its signature Phazon-generating abilities.
 * Because the Leviathan that hit planet Aether in Metroid Prime 2: Echoes was nowhere to be seen on the planet or in the Dark dimension that was created upon impact, the Leviathan Core was also not seen in this game. While the impact crater caused by the Leviathan can be seen on images of Dark Aether, theories suggest that it exploded during the dimensional split caused by the collision on Aether, which is supported by the fact that the Luminoth found no sign of the Leviathan when they left their shelters. It also explains why Phazon is prominently scattered throughout Dark Aether, while there were very low quantities found on Light Aether, some of which was stored by the Pirates. Also, the Emperor Ing is believed to have absorbed the Core, or the Core died from the poisonous atmosphere on Dark Aether. Finally, the area directly below the Sky Temple and the room where Samus fights the Emperor Ing contain comparatively large quantities of Phazon; the room might possibly be a loose approximation of a Core Room for the Dark World.