There's a big door at the far end of the room. Go and open it. It will start to open, and then close again, and you get a message saying, "The portal has been sealed. Choose your fate." Eight switches will appear in the room. One of these switches will open the door; the others will open small rooms at the sides with Dark Bishops in them. So far, I haven't been able to tell which switch opens the door; I simply pull all of them and kill the Dark Bishops.
Once the door is open, go inside, grab the Krater of Might and step into the portal.
The next room is an icy circular room with two pools and four switches. There are four different symbols on the switches. Now, look at the map! What does this room look like on the map? Find the switch with a symbol of this shape. Activate it, and the four switches sink into the water. This is one fourth of the puzzle in this tomb. (Note: I made a mistake here in the previous version of the walkthrough: activating this switch does not open the third room; the third room is always open. But this switch is related to solving the puzzle in this tomb to get to Menelkir. Thanks to Hiram Lester Jr. for pointing this out.)
Go to the third room now. Again, activating the wrong switch teleports in a gang of ettins to punish you for your mistake. :)
The third room is a triangular room. Again, there's a symbol on the pillar in the middle. Find the matching symbol on the wall and activate it to open the next room.
The fourth room is a circular room with a deep pit. There's a magic blue bridge that lets you get to the pillar in the middle, which has four symbols on it. Again, look at the map! What does this room look like? Find the symbol with this shape and activate it.
Now, go to the fifth room, which is another triangular room. By now, you should now what to do: activate the symbol on the wall that matches the symbol on the pillar in the middle.
The sixth room is another circular room. I don't think I need to repeat myself here. You should know which switch is the right one.
Another triangular room... same story goes for this one.
Another circular room... same old story. But this time, after you activate the switch, something happens. (Provided of course, that you've activated the correct switches in the other circular rooms.) Four new switches have appeared, one in each of the four circular rooms. You can press any of these four switches; all of them do the same thing: they activate four elevators in all four circular rooms. Step on one of the elevators, which will take you to:
Kill him, and a pillar will lower with the Sigil of the Magus on it, that looks like a mini-Bloodscourge. You need this, so get it. A portal will also open now, which takes you back to the Necropolis.