Based on the erotic game by Active.

Minase, a high school student, found a book of magic in an isolated room in his school. He started practicing black magic that has extreme sexual effects that benefited him and some of his friends. Intrigued, Minase got deeper and deeper into using the craft, not realizing the evils that will come forth. Eventually, the origins of the book was revealed, and so did the incident twelve years ago on the night of the Walpurgis, the night when the power of evil is at its strongest. After coming to his senses, Minase struggles to get himself out of the darkness that he had put himself into.

(Source: Anime News Network)