In Pokemon Legends: Arceus' "Disaster Looming" mission, players need to clear the trials of the three Legendary lake guardians of the Hisui region, Uxie, Azelf, and Mesprit.
Near the end of the main storyline, you'll be tasked with capturing both Dialga and Palkia. These Legendary Pokémon can put up quite a fight, so you'll need to come prepared.