I even reloaded the game before leaving Act II and tried to find that quest after that, but I couldn't find it. From conversations with my brother during the Christmas holidays I know that I did most of the stuff that it would take to overcome the shadow curse - the little shadow boy, his parents and apparently the leaders down in that freaky nightmare village... According to my brother I somehow missed Halsin in the Harper enclave, though. Which is weird, since he came with me, so I naturally looked in my camp where he wasn't.
Even after completing the Moonrise Towers I spent hours trying to find a way to lift the curse, but never figured it out.I was so annoyed with that that it caused me to Baldur's Gate III on hold for a couple of months.
I think you have to do that before killing Ketheric, before finishing the Gauntlet of Shar even. (Edit: There is a point at which you will be reminded to tie all loose ends before proceeding in the Gauntlet. You need to reload before that one. It's right before you enter into the Shadowfell.) In oder to find Halsin at Last Light you have to do something else first.
When you enter the main hall of the inn, there is a room to the right in which the Flaming Fist have gathered. In a bed on the far side of the room lies an injured man, whom the Fists have pulled from the Shadow Curse. He's the one you want. From what you wrote, you probably already have the item you need to get for him too. If you talk to Halsin - in your camp - after learning about the injured man, Halsin will go to Last Light.
