Wirt's Lost Treasure

[Draclvr]

I am a level 30 Assassin. I'm trying to get all the side quests done and am down to the last few. I have battled my way through the cave three times making it to the Great Unknown. I have killed all three mages each time and looted the libraries. But the gates will not open to the center and I am unable to complete the quest. Am I missing something?