When you defeated the Keymaster, a clue appeared in his room. Three obelisks appear on the floor opposite the entrance. They are Krok, Snake, Tree. They can be lit in any order, but only those obelisks must be lit.
It's been a while since I was there, but I think you need to walk out of the room where the six obelisks are. Go back and you will see four different obelisks hanging in the halls. These four have the clues as to which ones you light. (Sorry, I don't remember which ones they are.) Then you need to go back into the room and light only 4 of the six in that room.