This world is a world of black and white, a balance of yin and yang. When a good person dies, a good person is born; when a bad person dies, a good person becomes a bad person.
The demon kings in this world are all brave men who defeated the previous demon king. All they need to do is to do evil in the demon king's place until the next brave man comes and kills him, and then they can be freed.
It must happen in this world that the dragon-slaying warrior eventually becomes a dragon.
