From the Super Mario Wiki
Sammer's Endgame is the location of the final twenty-five rounds in the Duel of 100 in Super Paper Mario. It is not visited in the main story, as Sammer's Kingdom is destroyed by the Void before the heroes reach it. It can be visited after completion of the game to complete the Duel of 100. After the final Sammer Guy is defeated, King Sammer awards the heroes with Catch Cards of the partners from Paper Mario: The Thousand-Year Door.
Duel of 100