Here's how I worked around the bug. I had killed Balreth, his spirit, however, could not be contained. Even dropping the quest outside the portal to reset Balreth left me with an empty instance.

First, I found someone that had not killed Balreth yet.
Next, that person zoned in to fight with Balreth.
After my partner engaged Balreth he invited me to a group.
I then took the portal in to fight Balreth and we defeated him in short order.
Neither of us looted Balreth and Balreth's spirit went to the center where we could contain his spirit.
However, we still could not get out of the instance as the next portal did not show up. The workaround was to pull up the map and fast travel to the nearest wayshrine for about 60 gold. Then we could complete the quest.

I hope this helps anyone looking to complete this quest. Your experience may vary.

Edit: This worked as of 1 April in case there's a hotfix.