Mine showed the quest marker for all three even after all were dead. To complete the quest, you have to walk up to Halsin and talk to him one final time. For me he was in the main chamber with the hardest boss, because he came with me. If you left him at the cells, maybe he has gone back to the Grove.

I left my game yesterday after casually running out of the camp straight into a small army of goblins + a giant goblin, dying after a 45 min battle, then realising I'm going to have to be smarter to escape. Maybe there's a less-heavily-guarded exit.