The wind demon beasts have been scared of fire, but the frost energy of the snow stops normal flames. He's injected red oil and phosphorus into the ground, and then the flames will burn. The corpses that pile up will be the first defense line. Thousands of demon beasts are killed instantly. This kid's clever, he says, and it's saving them a lot of power. The demon beast horde is split. They're not attacking the walls of the city, they're retreating back to the south. The trenches are burning, and the trenches are going back south.