Adventure Mode: World of Light
World of Light (灯火の星, The Star of Light) is a single-player mode in Super Smash Bros. Ultimate, and the successor to the Adventure Modes in Melee and Brawl. The mode is accessed from the Spirits main menu item and revolves around the fighters teaming up with Spirits to defeat an "ultimate enemy" named Galeem.
Playing this mode is required to unlock the ability to train spirits in the dojo or have them explore for treasure.
In an interview with Reggie Fils-Aime, Reggie claimed that the mode was a way to "get a story and unlock some characters", possibly confirming that Adventure Mode is one of, if not the primary way to earn unlockable characters.
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