After the mysterious events in Hisui unfold, you set out to face one of the trials.
Go to Lake Verity
From base camp, use Braviary to fly to the guidance marker at Lake Verity. Approach Volo and your guide.
Enter the Cave
Go into the cave. Approach the Alpha Goodra and you will battle it. You can defeat it or try to catch it.
Answer the Questions
Approach the pokémon and answer its questions. Afterward, you get a key item.
Go Outside
Go outside. If you have completed the other two trials already, there will be a cutscene, leading to the next mission. If you have not finished the other two trials, you can choose to go back to the Ancient Retreat. You will need to do so before going on to the next trial, if you have not yet completed them all.
From the Ancient Retreat, approach your guide to choose the next location to go to.