Collateral Damage
Allow Balor to kill 15 of his own troops.
At the very end of Spoils of War you will need to fight Balor. There are several phases to this battle, but during the first section you need to lower his health until only a quarter is left on the health bar.
At this point he will start to pick up and drop his troops into the arena to offer a distraction. Instead of killing Balor, just allow him to kill his own troops until this unlocks.