Assuming you’ve downloaded the Genshin 2.1 update and completed the Ritou Escape Plan Archon quest, you’ll find Lunar Realm under the events menu.
Genshin Impact Lunar Realm is a new event and the first set of aquatic quests to crop up after miHoYo introduced the new fishing system as part of Genshin’s 2.1 update.
Lunar Realm is the best way to stuff your coffers full of scaly wealth.
Fish respawn at challenge spots immediately, unlike normal fishing spots, and you can catch up to 50 fish this way.