Priestess of a Godless Month

Two girls, Kurusugawa Himeko and Himemiya Chikane, who have a close friendship that is secret from their classmates, are suddenly confronted with their fate to be the Priestesses of the Sun and the Moon. When an ominous lunar eclipse occurs on their shared birthday, they are forced into a pitched battle against Orochi and his forces, who wish for the total annihilation of the planet.

But it's not quite that simple. Himeko has feelings for Chikane–and for one of the Orochi's "Necks," Oogami Souma, who somehow has the ability to resist the call of darkness, though not without cost. What will happen as these people learn to live their predestined roles?