|The entrance to Under Island|
Under Island is an abandoned shed belonging to the Ash Giants of Giants' Village, marked as Giants' Ruins on the map. It can be found on Hallowed Marsh's east coast, south of the entrance to Giants' Village on an island in the center of the bay.
Troglodyte Gem Wall
When you first enter, you arrive in a room overlooking a single room full of Troglodytes. The next room is blocked off by a stone wall with a gate on the far right side. In order to unlock the next room, a series of unique gems must be placed into the five slots in the wall to the left of the gate. This is a unique example of a Troglodyte Gem Door.
After placing these gems into the wall, the gate will open allowing access to the next room. Once you have opened the gate, you will have to clear the room of the five kinds of Troglodyte listed below. Once this is complete, a Trog Chest can be be looted between the two large red mushrooms. A Knight's Corpse can also be found laying on a rock to the left of the Trog Chest. Next to him lays the Fabulous Palladium Shield.