This quest is one of the series of missions you need to complete to unlock and access a special region hidden underneath Watatsumi Island called Enkanomiya. Before starting this quest, you need to complete a prerequisite named “The Moon-Bathed Deep.”
After completing The Moon-Bathed Deep quest, you will have in your possession an item called the Key of the Moon-Bathed Deep or Key of the Deep. Bring this Key to Tsuyuko, who is currently at the entrance of Sasngonomiya Shrine, and this will trigger the quest.
Tsuyuko will tell you that Enkanomiya is awakening, but their investigation is slow due to lack of manpower, and thus they need your help.
Follow Tsuyuko to the next location, where she will show you one of the five altar-like structures that activate a barrier to seal the Enkanomiya.
These structures need a Key Sigil placed on them to release the seal and restore the barrier to prevent ordinary people from going to Enkanomiya.
This will also be a trial system that will decide if you can access the main entrance of the underground region. Three of the Key Sigils have been placed on the altars, and Tsuyuko will need you to collect the last two and place them in the remaining locations.
Locations of the Key Sigils
One of the last two Key Sigils is found North of Watatsumi Island, where the Fin of Watatsumi is, while the other is inside the cave of the Heart of Watatsumi.
The Fin of Watatsumi will now be lurking with Ruin Sentinels, which you need to defeat first. After doing so, take the Key Sigil, which is just on the cliff wall.
Inside the cave of the Heart of Watatsumi, you will find Fatui Skirmishers, specifically two Hydrogunner Legionnaires and a Fatui Mirror Maiden.
Although there are only three of them, the two Hydrogunner Legionnaires can heal the three of them, making the fight a little more complicated. Use a Cryo or an Electro character on both so you can freeze and interrupt their attacks and healing skills.
After defeating all the enemies, you can take the Key Sigil from the fountain at the center of the platform.
Locations of the Altars
Go to the altars and place the Key Sigils to release the barrier. These altars are found on the middle rock arches of the Island.
From the Heart of Watatsumi cave, you go outside and glide to the location of one altar.
The other altar is easier to access by teleporting to the northern Teleport Waypoint and then gliding down the rock arch south of it.
After placing the two Key Sigils on their respective altars, head back to the Sangonomiya Shrine and talk to Tsuyuko so she can begin the ritual to open the first entrance to Enkanomiya’s trial.
This entrance is the pool at the center of Watatsumi Island, and the next thing to do is jump into the water and descend to Enkanomiya.
Completing this quest will also unlock the achievement “N-Thousand Leagues Under the Sea.”