Omega Cannon

The Omega Cannon is believed to be the Alimbic's "Ultimate Power." It fires a powerful but slow-moving ball of radioactive energy that has a timed detonation, traveling a significant distance before exploding, unless it comes into contact with an object, in which case it will detonate immediately. When Samus Aran fought Gorea at the Oubliette, she used this weapon to defeat the beast's final form, which was impervious to all other weaponry. The Alimbic Elders, although possessing it, refused to use it because of its horrible destructive power. Heavy gamma radiation emanates from the Omega Cannon, which suggests nuclear firepower.

What happened to it after Gorea was defeated is unknown. Samus may have kept it, the Alimbic race may have taken it back through the end game vision, or she may have handed the weapon over to the Galactic Federation.

Multiplayer
When playing the Oubliette level in multiplayer, this beam appears at the top of the stage. It is the only beam that appears in the map, and it is not permanent; after firing it once, it disappears from the hunter's beam selections. The beam differs from its use in the Gorea fight, in that it travels slower, has a larger blast radius, and does more damage. Once picked up, it is automatically selected to be used (most of the time), but the Power Beam or Missiles can still be selected afterward, saving the Omega Cannon until the hunter finds a better firing spot or target. Due to its incredibly large blast radius, it has the capability to wipe out hunters on an entire story of the stage. It will act as "killing anything" within blast radius, though this is actually doing 200 damage only. Interestingly, if playing the Prime Hunter game mode, it is possible to gain an affinity for this weapon while the hunter is the Prime Hunter. In this situation - and only this situation - the weapon acts exactly as it does when fighting Gorea.

Often, if Oubliette is selected for a match, the start of the match becomes a race to the top of the level to obtain the beam. After that, the match becomes a sort of "king of the hill," to protect the respawn area of the cannon.

Trivia

 * Once the Omega Cannon is obtained during the Gorea 2 encounter, Samus cannot use the other sub-weapons she obtained during the mission. It is unknown whether Samus lost these upgrades or if they were stacked with the Omega Cannon.
 * Although the description of the Omega Cannon states that it in some way harms its user (apparantly quite seriously), Samus seems to suffer no ill effects from firing the weapon repeatedly at Gorea. It may be that the Omega Cannon is only harmful to Alimbic physiology, that whatever damage it causes only maniphests itself after a significant amount of time has passed, or that Samus simply did not use the Cannon for a long enough period of time for its effects to become noticible.
 * The Omega Cannon is the only weapon in the game that differs in mechanics between single and multiplayer, not including affinities.
 * Unlike the other sub-weapons Samus collects, the Omega Cannon has unlimited ammo in single player.
 * The Omega Cannon is the only weapon obtained in the Alimbic Cluster that does not have any particular technology that puts it into use.
 * The Omega Cannon is perhaps the most powerful beam in the Metroid series (alongside from the Hyper Beam in Super Metroid and the Phazon Beam in Metroid Prime.)
 * The Omega Cannon does not change the color of the markings on any hunters gun.