Back in the day, the Oni were the evil themselves in Feudal Japan, it is said that Oni's came from Devil gates just to decieve (and often eat!) people.

Nowdays, Onis are used to fend off the bad luck that anybody could have. So, you don't lose anything by having one of them in your weapon, just in case. You don't want to miss a headshot just because you didn't had one...right?

For it's Holo variation, you have the magical rainbow-colored Oni, as they also came in Green, Red and other variations through Japan's history.