Pokémon Legends: Arceus is set in the historical region of Hisui.
The game takes place long before the events of Pokémon Diamond, Pearl, and Platinum. Hisui is the earlier name for the land that later becomes known as Sinnoh, making the game a historical predecessor to the fourth-generation titles.
Players join the Galaxy Expedition Team and work to create Hisui's first Pokédex. The story is based around the settlement that becomes Jubilife Village and includes expeditions into areas such as the Obsidian Fieldlands, Crimson Mirelands, and Cobalt Coastlands.
Hisui is not simply a separate modern region unrelated to Sinnoh. It is the same broad setting at an earlier point in its history, with different settlements, customs, and Pokémon forms. The game introduced Hisuian forms, including regional variants of Pokémon such as Growlithe, Voltorb, and Zorua.