The Demon King (Nightfall Saga, #3)

But pursuing him are humanity's best hopes: Olive Paper and Darin Bales, whose legendary parents brought demonkind low once before. Olive and Darin will not rest until the demons are defeated, and so relentless is their hunt that they have followed Alagai Ka across the sea to a strange new land.

There they discover a culture unlike any they have never known, where demons live alongside humans as servants and companions. And there they meet the demon's masters–including a seductive prince who is drawn to Olive – who seem unable or unwilling to understand the danger they are in.

Because no human is safe from the Demon King's thirst for war – and every human must join the battle against his kingdom of death.