When the battle is over, talk to Nevalle once more, then enter the Keep. Aldanon will tell you that Amon is missing. Take your party and go to look for him. You'll be automatically taken to Shandra's farm, where Amon is going to battle his former prisoner Koraboros. Defeat the monster and talk to Jerro. If you can convince him to feel guilty about Shandra (there will be influence checks on Amon himself and Shandra), you'll get some XP and a neat amulet. Go back to the Keep.