Encountered a bug in the quest to
lift the shadow curse
in act 2.

After triggering the quest by
waking art with the lute
Halsin walked off to the lake shore.

I then left the area, going to the
Mausoleum
and
Moonlight Tower
.

Upon returning, Halsin is no longer there. He can still be found at the act 2 camp.

Redditors have found a workaround ( https://old.reddit.com/r/BaldursGate3/comments/162yqd5/halsin_not_at_lake_shore_bug/ ) but it doesn't work for me. The subject refuses to talk to his killer.