Принц-демон Дзэнки: История демона

In Kanoesaru village every 60 years mysterious murders happen. Every time seven 17 year old girls are killed. The villagers believe it is the work of a demon who once saved the village so they keep quiet about it. However the current head of the Ibuki family calls an exorcist to save her 2 daughters. What is the truth behind the murders in Kanoesaru village? (Source: AniDB)