Dark Cannon is a device wielded by members of the Subspace Army during The Subspace Emissary in Super Smash Bros. Brawl. With a single hit from its black, arrow-like laser, any character will be reverted to his trophy form. Dark Cannons are mostly used by Wario and Bowser; Ganondorf also uses one late in the story. Even Tabuu wields a Dark Cannon as one of his attacks.
In this world, those who fall in battle return to their forms as trophies. This Dark cannon is powerful enough to turn a fighter back into a trophy with a single shot. Originally created by the Subspace Army, they were then given to Bowser and Wario for the purpose of collecting fighters who exist in this world.