Dai Seirei no Keiyakusha ~Jashin no Kumotsu, Saikyou no Boukensha e Itaru~

Marc, a boy born with white hair, was persecuted by his village because of his appearance. He was fated to sacrifice his own body to an evil god dwelling in another world on the "Day of the Covenant," an event that occurs only once every hundred years.

When the Day of the Covenant finally arrives, Marc is sent to the otherworldly realm, where he comes face-to-face with the evil god, Nerodias. Marc trembles in terror before the horrific, monstrous entity.

However, it seems the legends got it completely wrong...