Chapter One: Who Is He?

Muichiro knew that the Final Battle was arriving, so did his fellow Hashiras. Thrown into an intense battle against Upper Moons, he came across the strongest one of them all.

Upper Moon One.

His power and aura was terrifying to the youngest Pillar, but this did not waiver his determination.

That was until he learned that he was the descendant of that monstrous swordsman.

And about the six-eyed monster twisted view on him despite just having met him.

Or, a Platonic Yandere Story about Kokushibo where Muzan won. Genya and Muichiro are stuck with UM1 and are transformed into demons.