NOTE that for some reason in my gameplay, the door leading into the antechamber
wouldn't open again after I had killed Raven. If this happens to you, you'll
have to do some clipping to get through. Left-Strafe into the corner where the
stone door meets with the stone frame around it, and play around with the turn
keys, left and right. It may take some trial and error, but you should pop over
to the other side, and the door will the open