Kokushibo had left his humanity behind, including his family and his brother, Yoriichi, who had died of old age just before they could fight. He cried for just three of his six eyes before losing his humanity. He hated Yoriichi, it started with envy about him being the strongest but now it had switched to hate, unlike the fear that Muzan had of him.

But that didn't matter, Yoriichi was dead, and now 500 years later, there were a new generation of newer and way weaker slayers. The Hashira weren't a threat to Kokushibo.

One night, Muzan's plan was put in action. He went to the Ubuyashiki mansion to kill Kagaya and even if he was tricked, the mansion exploding and him getting trapped by an unknown demon art and weakened by Tamayo, he had a plan. All the slayers were sent to the Infinity Castle, where all the Upper Moons that were left (Kokushibo, Upper Moon One, the strongest, Douma, Upper two, Akaza, Upper Three, Nakime, Upper Four and Kaigaku, new Upper Six) and some demons were.

Kokushibo wandered through the halls trying to find some slayers, he killed some low rank ones. Until he found someone in a wide area, someone that looked way too much like his younger twin, Yoriichi...

"You..."

Kokushibo mumbled, keeping his cool. He knew Yoriichi wasn't alive, so who was this person? A descendant from Yoriichi? But that was impossible, right?