I just got to this point with my Ancients paladin as well and experienced the same issue. In fact, Elder Brithvar went hostile immediately after the fight for "killing a guard" and I had to kill him, too! My character is now an "Enemy of Justice" in Grymforge and instantly attacked by any remaining duergar.
I sucked it up and paid my 1,000gp to get my powers back. The scenes with the Oathbreaker Knight in camp would be really cool if it wasn't due to a bug.
