Kekkaishi, Vol. 15 (Kekkaishi #15)

By night, teenager Yoshimori Sumimura is a "kekkaishi"--a demon-hunter who specializes in creating magical barriers around his prey. By day, Yoshimori's got other demons to battle: an addiction to sweets and a seriously crotchety grandfather! Yoshimori's pretty neighbor and childhood friend, Tokine Yukimura, is also a kekkaishi, but their families are feuding over who is the true practitioner of the art.

A vicious ayakashi is growing inside Yoshimori's school, feeding on the energy of the Karasumori site... If the ayakashi stays, it will grow ever more powerful. But if Yoshimori defies orders and attempts to dislodge it, it could ravage the entire city...!