Shiratsuyu no Kai

The protagonist is a girl who has been invited by some boys to spend a night telling ghost stories in an old shrine as a test of courage. In the middle of the hundredth story, they discover a hidden door in the shrine. After descending an underground path, what they see at the end is... Legend, a guardian deity, sacrifice. They had thought this was all a forgotten old tale. But when the sacrificial markings are left on the girl's body, they are once again visited by the guardian deity. Being called by the fate of their own blood...