Demon invasion.

There is a bug/exploit in mission 4. If the Cave Dweller units from the hero's specialization defeat all enemies with a Fireball spell at the beginning of the battle, and the battle ends, they remain and are added to the army (interestingly, this does not work in battles against demons). Out of curiosity, I completed the mission using this bug, and by the time Duncan arrived, I had 10,000+ Cave Dwellers.