The Tail of Watatsumi is a subquest in the Moon-Bathed Deep quest in Genshin Impact. Find the area to complete the quest is easy just open your quest list, then travel to the waypoint when you select the Tail of Watasumi quest under the Moon-Bathed Deep entry.
Just like the other shrines, this one will be sealed. You will need to open it to interact with it. To do so, read the tablet to the left of the Shrine. You find out that you need to light all the nearby lanterns.
Head down to the bottom of the slope and take care of the Cicin Electro Mage that is down there. After that, interact with the Thunder Bought beside the tree, then walk back up the slope, interacting with all the lanterns. If you stand close to the lanterns, the Electrogranum you got from the Thunder Bough will spark and light them.
When all of them are lit, you will notice a group of Fatui spawns at the bottom of the slope. Run down and eliminate them, and only then will the shrine become unsealed, allowing you to grab the pearl from it. Once that is complete, and you have all four of the shrines unsealed and the pearls obtained, you can complete the Heart of Watatsumi quest.