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This world consists of two dimensions joined as one, like a mirror. The first is the world in which the humans live, Assiah. The other is the world of demons, Gehenna. Ordinarily, travel between the two, and indeed any kind of contact between the two, is impossible.
However the demons can pass over into this world by possessing anything that exist within it.
Satan is the god of demons, but there's one thing that he doesn't have, and that's a substance in the human world that is powerful enough to contain him!!
For that purpose he created Rin, his son from a human woman, but will is son agree to his plans? or will he become something else...? An exorcist?

Credits to HorribleSubs!