Bible Black.

Taki Minase, a high school student, finds a book of black magic in the isolated storage room of his school. He begins practicing black magic, which has extreme sexual effects that benefit him and his friends, until evil comes forth from the dark craft. Eventually, the origins of the book are revealed, as is the incident on Walpurgis Night twelve years prior, when the evil force was at its strongest. After coming to his senses, Minase struggles to get himself out of the darkness into which he has put himself and his best friend and secret love, Kurumi Imari.