I just had this problem. By the time I was done killing the priestess…the dude was gone from his cage….yet the quest never indicated this. My next step in my quest was to release him….but I couldn’t interact with his cage. I had to leave the instance and start all over. We even tried dying and restarting from a checkpoint.
Worked the second time, as we were careful to heal the dude in the cage whenever the Priestess got him with her AoE. I reported it as a bug.
I think he should either be invulnerable, or we should be aware that we need to heal him….and if he dies, the quest should update and let us restart from a check point.