I got idea form the uchiha rap and corpse demon
so I though why not the person your playing as is a uchiha but somehow had a choice between 3 choices and he chose to have all the mangeko sharigan patterns and they switch everytime in combat without the side effects like blindness and high chakra usage just blood from the eyes
the backstory of this being that that the sharigans eyes itself like the users/in game characters eyes are black like madara and corpse demons character's mangeko eyes are black so I took inspiration from that but the ACTUAL BACKSTORY is that he was exiled from the uchiha clan for being to weak and a senju woman (you should see where this is going :P) secretly nursed him back to health so in return he said that he'd protect her for life until he dies but instead she decided that he would be better on the front lines with the senju and that's basically the story
it takes place in the waring states period