Kandachi no Ame, Oni no Uta

Onikazemura... A small village surrounded by mountains with a population of around a mere 2,000. Here, there was a practice since ancient times of offering the mountain oni a young female sacrifice. The protagonist and sacrifice Hikari (name changeable) was to be offered to the oni deep in the mountains once the summer thunderstorms- the "Kandachi"- began to rage. Before she was to offer herself to the oni though, Hikari had to birth the next sacrifice. For this reason, she spent every night in Tatsuki's- the village leader's son's- bedroom. During that time, she met Homare, a young man who also lived in Onikazemura. "I'm a sacrifice... So I don't need love nor do I need friends." That's what Hikari had thought, but... [Translation of DL Site's synopsis.]