A long time ago, there lived a jorougumo named Hatsune who gathered enough celestial energy that she was able to take a human form.

One day, a mysterious monk came to Hatsune's nest and nearly annihilated her for taking a villager's life. Severely wounded, Hatsune went into hiding. To heal her wounds, she decided to make Yaesaka High School her new nest and absorb energy from the students attending there. Hatsune absorbs energy by eating the flesh of (mostly male) students or through having sexual intercourse with them (preferably with females).

[From Wikipedia]