What is a two faced oni mask?

Answer 1

Oni, which we believe is a unique phenomenon that is embedded in deep and unknown levels of the human mind, plays a relevant role in our lives at times of transition. The widespread recognition of the Oni phenomena is often associated with turning points in history, and seems to reflect meaningful phases in an otherwise chaotic life. The devil (Oni) mask is to be oldest.

As same as Fierce God Style the most of devil masks are used for protection from evil spirits.Devil masks serve two purposes; the first to portray the devil and create disasters or cause the plague, and conversely they are used to ward off evil spirits.