Story to Get Thundurus – Pokémon Legends: Arceus
You will only be able to unlock this request after you have reached a certain point in the main story and you've added Clefairy to your pokedex.
The professor has a blackboard in his lab where he writes interesting things you can learn through there.
To officially begin the quest, you must speak with Astair himself in the basement of galaxy hall.
The clefairy appear at nighttime in the Fabled springs of Coronet. This location is also where you can add clefairy to your pokedex to unlock the request.
To make sure you don't get lost along the way, make sure you toggle on quest markers by pressing a while hovering over this request on the request list.
The night of a full moon is the only time that you can enter the spring.
During a full moon, you'll be prompted to investigate. You will find out that the event started.
When you watch the cutscene, your character will notice a bright light on the ground.
You have to check it out to pick up a moonstone.
The moonstone is a special item that you use to evolve clefairy into clefable.
You must report to Astair to officially close the request.