Through the door, you will reach a tiny room with a lever on both sides of the door. This is an elevator room -- pulling the lever will lower you way down to the bottom, where another door waits. This door requires that you have the key to Ghariks Laboratory. You can get this key from the Silver Helm Outpost on the Misty Islands. In the next room (which is a nasty fight), go through the wall on the far door, and then up the spiral ramp around the edge of the next room. Up there, you need only fight your way across one more bridge and then through the last group of warlocks in the room on the far side. The chest against the far wall contains the Hourglass of Time needed to complete quest 12.