Catching Tornadus - Pokémon Legends Arceus
After beating the game, go to the Galaxy Hall. Cogita will be there and will give you the mission or request 94: Incarnate Forces of Hisui.
Tornadus is located in Alabaster Icelands, in the area of Bonechill Wastes. You must go there at night. I killed some of the Glalie before starting.