Omega Cannon

The Omega Cannon is 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 destroy 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, suggesting that it could be a small scale nuclear weapon.

What happened to it after Gorea was destroyed is unknown, though it is likely the Alimbic spirits that rescued and returned Samus to her ship and then took the weapon in order to prevent any more destruction by its use.

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 your beam selections. The beam differs from its use in the Gorea fight; it travels slower, has a larger blast radius, and does more damage. Once picked up, it is automatically selected to be used, but the Power Beam or Missiles can still be selected for use, saving the Omega Cannon until the player finds a better firing spot. Due to its incredibly large blast radius, it has the capability to wipe out players on an entire story of the stage. It will instantly kill anything within the blast radius, regardless of health. Interestingly, if playing the Prime Hunter game mode, it is possible to gain an affinity for this weapon, while you are 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 regeneration area of the cannon.