After mindreading the goblin, you need to read the book he was thinking off in the bookcase on the other side of the room (you might need to scroll down the list). After that, you need to go up a level and defeat an opponent to get the key to the chest in the secret room the book revealed, which will give you the name of the demon you need. The door will then be open on the third floor (but finish off everything you want to do in Broken valley and Sentinel Island before going past that point, as most open quests will fail and most NPCs will be unavailable afterwards).


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