Omega Weapon

The Omega Weapon (オメガウェポン) is a form of the Kingdom Key that is changed into the Omega Weapon with the Pandora Gear equipped.

Trivia
Its name is originally from the recurring superboss in the Final Fantasy series

Ironically enough, while Pandora Gear+ grants all the other Organization members their most powerful weapon, it provides Roxas (and by extension Xion) with lackluster stats.