After the events at the Temple of Sinnoh, you now explore the myths of Hisui, for the sake of the Pokédex.
Get the Other Starter Pokémon
You can now talk to Professor Laventon in Galaxy Hall in Jubilife Village to get the other two starter pokémon.
Meet with Volo
Go to Heights Camp in Obsidian Fieldlands and approach Volo.
Go to Grueling Grove
Go northeast to Grueling Grove and approach Volo. Battle the level 60 Alpha Vespiquen. You can try to catch it, or just defeat it. You will get the Stone Plate.
Go to the Ancient Retreat
Go back to Jubilife Village, and from there, talk to Ress at the Front Gate and go to the Ancient Retreat. Go into the building and talk to Cogita. Ask her about each topic to start the following missions: