Instructions
Travel to the entrance of the temple, if you aren't already there. Head north up the stairs, defeating enemies as you go, turn west and open the door. This location is marked on your map as "Cultist Chambers."
Make your way through the rooms, defeating any enemies you encounter and being careful not to trigger traps.
Locate and open the ornate chest in the last room. This chest contains the black pearl, a quest item you will need to progress further, and the key to the south-east chamber.
Return to the entrance of the temple and defeat the enemies that arrived while you were gone. Head to the south-east chamber door and open it with the key.