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The Underdome once acted as a coliseum where Hades pitted fiends of all kinds fight in the Underdrome. However, Zeus was against such senseless violence and closed it down, hiding it beyond the Lost Road. Upon learning from Pete that the Keyblade could reopen, Hades kidnaps Megara and traps her in the Underdrome, forcing Sora to unlock the Underdrome to rescue her. Hades sent the heroes off to fight the Hydra, while he plans to restart the games.
During their second visit, Sora, Donald, and Goofy returns to the Underworld in their quest to find Organization XIII's hideout. Hades then forces the trio and a disheartened Hercules into the Hades Cup, manipulating Auron into being his assassin. However, Sora and co. manage to break Hades's spell and help knock the villain into the River Styx.
After Hades returns to the Underdrome, he decides to hold tougher tournaments to kill Sora, Donald and Goofy. When they manage to beat the Hades Paradox Cup, an infuriated Hades is kicked by Pegasus as the team celebrates their victory.
The Underdrome hosts eight tournaments in all; four regular cups, and four "Paradox" cups that can only be entered by talking to Hades; the first three Paradox cups are merely reruns of their normal counterparts, except that the opponents have been empowered, while the final Hades Paradox Cup is a lengthy tournament taking elements from all previous cups.