Can't finish "Repentance" quest from Sebastian DLC

I started the Sebastian DLC and killed the mercenaries for him.  Then in Act 2, in the quest "Repentance," we go to the Harriman mansion to find out why his family was killed.  I made it through the mansion, went down to the catacombs and killed the demons, then made my way to the last room which should culminate the quest.  The game autosaves; then, when my team moves toward what looks like a Desire Demon, everything freezes up.  I see the word "loading" on the screen (I don't recall seeing that in any other part of the game) but it just keeps spinning its wheels.  What gives?  What do I do to finish this quest?

I downloaded the DLC from the Bioware site.  My gaming desktop is not even a year old, so it shouldn't be system resource issue.  Any guidance?