(Literally Nakime during the Infinity Castle fight but in an au where Yushiro doesn't control her and she just backstabs Muzan lmao.)

You watched as Nakime, the recently appointed Uppermoon 4 after the previous (Hantengu) died during the battle in the swordsmith village, use her biwa to move platforms from within the Infinity Castle to try and crush Obanai and Mitsuri. You were suddenly pulled backwards and fell down to where Tanjiro and Giyu were having a standoff with Muzan, the king of demons. His appearance was completely changed, as his hair was white, his body was covered with mouths with rows of sharp teeth, and on his back were tentacle-like appendages with blades on them.

Muzan notes that Nakime has "killed" both Obanai and Mitsuri, before he stops. The two Hashira were pretty far away, but they were standing up. They were still alive! Muzan looks at Nakime and watches her strum the biwa, and the most unexpected happened: Nakime sends a platform crashing into Muzan. None of the demon slayers or Hashira knew how or why, but Nakime has betrayed Muzan.

Muzan: NAKIME!!! WHAT THE HELL ARE YOU DOING?!

Nakime doesn't respond as she continues to have that stoic expression on her face as she strums her biwa again, teleporting more platforms and even demon slayers to help defeat Muzan. Muzan then destroys the Infinity Castle, sending everybody out into a town where the Kakushi had already evacuated every civilian there. Muzan looks at you, and then the demon slayers, and then Nakime. His face was filled with rage and fury as his best Uppermoon just turned her back on him. All the demon slayers and Hashira charged towards him, beginning the final countdown until the sun rises.