The idea is that kiba's sword birth and blade blacksmith sacred gears mutated and became a subspecies and merged toghether into a single sacred gear: unlimited blade works. Maybe i will trow in lancelot's knight of owner and eternal arms mastery(i don't know the diference between those two...) as well
Now, i dislike kiba, so i will probably have mc be either a reincarnator or at least be amnesiac when he becomes rias' knight