From the Super Mario Wiki, the Mario encyclopedia
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After the events in The Glacial Peak, the Ice Climbers land on a mountain, alongside where Marth, Ike, Pokémon Trainer, and Lucas stand. Many Primids appear, being created by Shadow Bugs. Marth pulls out his sword, ready to attack, but seen in the background are Mario, Kirby, Pit, Link, and Yoshi, all taunting. The five jump off of the mountain, and the player picks four out of the five to battle in a multiman-brawl.
Later in the story (after the events on the Battleship Halberd Bridge and in The Subspace Bomb Factory), it is presumed that The Canyon is the landing place of the Halberd and the Falcon Flyer, and where the heroes set off to find Ganondorf and Bowser in Subspace.
Stage Clear Reward