There is a portal that opens at some point in the quest- when you go into the fort near the end-

After the quest the portal stays open- I went through the portal again and there was no way to get out of the small area that it takes me to without loading a save game. (after the quest is finished)

Also, Sanguine himself will stand outside of the portal but he won't really have anything to say, and so I'm not sure if he is supposed to be there.

Lastly, once the portal opens, regardless of wether the quest is finished or not, there is a room that is located underneath the portal, where there is a chest and a lockbox. But they must be directly under the portal because if I try to approach them, I go through the portal.