The Trial of Lake Verity Walkthrough - Pokemon Legends: Arceus
Travel to the Obsidian Fieldlands and go to Lake Verity.
Go inside the Verity Cavern and defeat the Lv58 Alpha Hisuian Goodra.
After you complete the battle, talk to Mesprit to finish the mission.
You have a few options here. They won't affect the outcome of this mission.