Namahage are a special kind of Oni that, while fearsome in appearance and behavior, are actually servants of the gods.
They come down from the mountains and go from door-to-door in order to ensure children are behaving and that newlyweds have kept to traditions. They often carry large blades, and while they frighten people, they rarely actually harm them.
They do have authority to harass humans, however, usually so as to ensure humans are both frightened into obeying the gods and also to make those who do evil confess their wrongdoings.