Meijou Shigatai Kanojo to, Ano Koro Okubyou datta Watashi no Hanashi

Mayu, a shy high school girl, is rescued by Rei, a mysterious beautiful girl who controls monsters, and falls in love with her. Little by little, she becomes strongly attracted to Rei, who repeatedly performs murderous rituals, using monsters for her own purposes... What is Rei's purpose? Will Mayu's wish come true?