I wish I could help. I never actually found him; he just showed up at the Grove after I wiped out everything in the goblin camp and went back to sell everything before actually looking for him.
I will say that sometimes you can just reload a game (doesn't have to be an earlier save or anything just save and reload where you are if you want) and it fixes a ton of scripting related bullshittery like this. Any time I notice a script failing to fire (like a line of dialogue not playing, or the enemies get stuck just standing around in combat never actually taking their turn), I quick save, quick load, and it's usually fixed.
I just did this again on my second playthrough, and he didn't show up. So I went back and looted until I was full, and he was just standing at the gates when I went back to Aaron to sell.
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.
He worked as intended for me. Had already killed one of the three "bosses" (Gut) before freeing him from his cell. Took Halsin with me, next killed Droz, and then finally, located the drow. After she went down, he immediately triggered the quest ending.
I'm guessing that if you DON'T take Halsin with you, you're going to have to go back and talk to him to complete the quest?