The world is round beneath the heavens and square upon the earth. There are nine layers of heaven above and ten hells below, making nineteen realms in all. Those who dwell in heaven are called gods, and those in the ten hells are called demons. The space between heaven and earth is the mortal world. Since gods and demons cannot adapt to the laws of each other's realms, they often fight in the mortal world, leaving its inhabitants to suffer. Gods mingle among humans, while demons disguise themselves as beasts and birds, though neither is human by nature. Gods and demons are innately powerful, but they lack humanity's capacity for insight and learning. Yet one human boy emerged from the mortal world, fought his way up through the nine heavens, and unified the ten hells. From then on, heaven and earth were united, and war vanished from the mortal world.
