This is what happens if you ignore curing the lands of Act 2 from the shadows, Halsin feels oblidged as a druid to stay behind and go about the hassle alone.
Did you have a scene play out where you see him for the last time when you depart for Act 3?
Do you have any mods installed?
I did not ignore it, I did the quest. The portal, everything. The little spirit even talked to me the night Halsin had disappeared. As a consequence, I can only assume it concluded correctly. It's literally just Halsin.
My last scene with him was the dialogue where he officially -- in theory -- joined the group. That was right after the portal and merging the Thaniels. I can't remember if that's supposed to be his last one.
I still need to kill the rat man, I haven't left for the third act yet. I'm on video game break today, though. I might see tomorrow if there's an extra scene If I leave for the city. I have some mods, but nothing drastic that could explain this.
In my experience with the game, he's either:
A) somewhere specific and I don't know it
B) got some flag triggered or not triggered and it's causing issues
C) ??? absolute somewhere glitched in a tree nonsense
I'll see if I can find something in my journal tomorrow. There's usually hints with companion characters. Still, I expected him to be there for the post-K. death camp night, since everyone and their grandma was there.
As a warning, I will not invest several hours into investigating this. I already did that too often in Act 1 and am behind on schedule.