Hisui was the historical name of the Sinnoh region in Pokémon Legends: Arceus.
The 2022 Nintendo Switch game is set centuries before Pokémon Diamond and Pearl. In that earlier era, the land is called Hisui, and the settlement of Jubilife Village is being developed by the Galaxy Expedition Team. The region is based on Hokkaido, the northern Japanese island that historically had the name Ezo before later changes.
As the story progresses, Hisui becomes recognizable as the future Sinnoh region. The game includes familiar locations, characters, and Pokémon in earlier forms, including regional variants and new evolutions. Its central goal is to research Pokémon and complete the first Pokédex of the region.
Hisui is not a separate modern region unrelated to Sinnoh. It is the same land in an earlier historical period, which is why the game functions as a prequel to the fourth-generation adventures.