I got to the end of this dungeon. Mercer Frey appeared, stole the eyes, Karliah & Brynjolf started fighting and now I'm supposed to take care of Mercer. The problem is once the sh** hits the fan, Mercer completely disappears. I can hear him taunting me and I can see his red dot on the compass, but he is nowhere to be found! Any ideas on how to fix this? Or is there some completely obvious lever I've missed that I'm supposed to pull?