Series concept: Humans and demons are two sides of the same coin, as are Assiah and Gehenna, their respective worlds.
In a desperate attempt to conquer Assiah, he sends his son instead, intending for him to eventually grow into a vessel capable of possession by the demon king.
Ao no Exorcist follows Rin Okumura who appears to be an ordinary, somewhat troublesome teenager—that is until one day he is ambushed by demons.
However, Satan, the ruler of Gehenna, cannot find a suitable host to possess and therefore, remains imprisoned in his world.