Actually, there was abit of cut content for Halsin. He was the one that killed Isobel with the "Sorrow" glaive, which then lead to Ketheric cursing the land.

It was described in the "Druid Notebook" in Nettie's Chambers and why the Glaive had self damage.

Presumably, due to his guilt, this was the reason why he wanted to go with us.