Purgatory of Cartagra

At Seirei Academy, a series of mysterious demonic ceremonies have been taking place and the transfer student, Yuriya Yuuki is suspected by his classmates. One evening, from his dormitory window, Yuriya sees a girl wandering outside and follows her. Once outside, he encounters a book which allows him to summon the demon Belial, who invites Yuriya to enter into a contract with him