The avernus DLC hope just died, Swen said in the GDC( there is a IGN article about it) that there won't be any expansion or DLC for baldur gates 3 so that route to fix kalach is gone. Now I think the only way karlach gets a good ending is in a definitive edition unless by some miracle they change it in the base game(I don't think is going to happen but you never know).
