North of Hyrule Castle, you find a rift where the sanctuary used to be, and Tri can open an entrance there.

Find Tri's Friends, Part 1

First, enter the Sanctuary and defeat the dark Moblins. When you defeat the dark Spear Moblin, it drops a glowing object. Check on it to save one of Tri's friends.

Find Tri's Friends, Part 2

Exit the Sanctuary and go east. Jump on the sides of the trees to go up to the graveyard area. Defeat the ghosts and you can learn the Ghini echo.

Go south from the graveyard, defeat more Ghinis, then bind to the grave that has a glow around it. Pull on the grave, then check on the glowing object to rescue another of Tri's friends.

Find Tri's Friends, Part 3

Go west and you find an area with sideways graves. Go to the southwest edge of this island, use an echo to reach the higher ledge to the north, then bind to the glowing grave. Pull on it, then jump onto the highest part of the island and fall down onto the grave next to the glowing object. Check on the object to find the last of Tri's friends.

There will be a cutscene. Tri's level gauge increases, and you receive 2 Might Crystals.

You can now talk to the person in the Sanctuary to hear some rumors, and you can play an acorn-finding game with the person outside.

You can light the two torches next to the large gravestone and go down the stairs to find an area with ghosts, as well as a stuffed toy that lets you learn the Stuffed Toy echo.

Then go west and through the door. Defeat the Ghinis, then create a Ghini echo and target the switch behind the northern wall. The Ghini will hit the switch. Go through the east door.

A lot of Ghinis, and a large Ghini, will attack. Use Stuffed Toy echoes to distract the ghosts, then attack them with monster echoes or swordfighter form, but be careful not to destroy the stuffed toys. After the battle, learn the Ghini Lv. 2 echo. Then go through the east door.

Open the chest to get a golden egg. Then step on the button to open the south door. Go through, then go south to go back outside.