Talk to Brynjolf, then head to Irkngthand. Head to the top of the ruins, watch out for bandits (which you can kill if you'd like). Sneak through the dungeon so as to not trigger the Sphere Guardians. Make your way through to a room with flame traps. Just follow through the flame trap as if it were a revolving door and you'll be fine. Take the lift.
Now you will have some company. Go through the door and down the hall to findMercer attacking a Folmer. Hit both switches over looking the camp and then head through the door. Continue along until you get to the sanctuary. You will find Mercer and have to fight him alone. He's tricky so watch out for his invisibility . Try freezing him or hitting him with stream attacks to find out where he is. Once he's dead, take the skeleton key and Eyes from his body and wait for the water to flood the room. You will go out through a hole in the ceiling here and exit through a small passage.
