In a prosperous city, there are many races- which are beyond the understanding of humans- hidden in society. The strength and the prosperity of these races not only depends on their own growth, but also on human belief and worship of them.
However, as more people in modern society believe less in them, the more their numbers and strength diminish. Although there are still some powerful and ancient beings, they can only try their best to hide in corners of the world or pretend to be human.
On the other hand, there are other races with dark purposes who call themselves the "Devil Hunter Association". In order not to be discovered by the demon hunters and not targeted by humans, various races worked hard to integrate into human society.