Author: Nevermind, found out I needed to have Agarth enter first, and he was for some reason waiting for me down the road.
You can apprently bug this quest if you fast travel to sell/salvage items in your pack before actually entering the House of Vengeance. Agarth will vanish when you fast travel back, perhaps clipping in the rocks near the door. I ran down the road a short distance and kited some Tuatha back to the area near the door, and Agarth reappeared to help me fight them off, which allowed me to gain entrance. Players may have to revert to an earlier save if this doesn't work, however. -Vale