Way of the Oni

From publisher blurb:

Gozu the Night Terror, an Oni of incredible power, once traveled the Iron Cage Summit in the guise of a legendary master monk. Using this guise, Gozu founded the Way of the Oni and taught others a unique tradition born of his own supernatural skill. Unfortunately, training his students could not subside the Oni’s hunger, and after preying upon a small city below his monastery, Gozu was discovered and imprisoned in the Mask of Oniborne. This legendary artifact serves as his prison to this day.