Maybe you can separate the group member that Halsin is attached to (looks like he is attached to your main) from the group.
This way Halsin should stay with your main, while the rest of the party can head to the underdark.
There is a waypoint at the entrance to the underdark if you havent been there yet

Separate a party member by pulling their character portrait away from the group until the chain breaks.

You could also try to kill him, maybe?

Last edited by kurg; 19/10/20 03:24 PM. Reason: Add spoiler tag