I think I might have some ideas for Hibiki world, to add to RK's
my idea is that basically there were 7 Onis that participated in a war against the Makamou. The war ended when the Makamou, Gyuki was sealed away. The Makamou that were left simply had no more intrest in fighting and most went into hiding.
Amongst the Oni that participated, only three remained(Hibiki, Ibuki, Zanki) the three went on to create a school to teach apprentices how to fight like Oni-should the Makamou return.
Cue several hundred years later, Hibiki decides he suddenly wants to retire, and pass on the school to one of his pupils, the title is up for grabs by anyone who proves themselves worthy to Hibiki's eyes, this also includes Hibiki's star pupil: Asumu(Who'll be using Kiriya). This erupts into a war between the current Zanki and Todorki, causing the school to be divided.
The Makamou steadily become more and more active, Kabuki of the renegades(who is actually half Makamou) wants to unseal Gyuki and beleives the way to do so is if he has all three scrolls of the Oni riders. With the fight between the schools proving the perfect distraction, he/she moves with his/her plan.