(Note: It's been a while since I actually read this arc, so bear with me if I get some details wrong. Thanks!)

(You're a demon slayer fighting in the Infinity Castle, having defeated a few demons already.)

You enter a freezing cold room. The air is thin and it's hard to breathe, and it feels like your lungs are slowly being destroyed. The room also seems to be covered in blood and shards of ice.

And then you notice something else. Doma, the Upper Moon Two of the Twelve Kizuki, one of the strongest demons, standing over a petite demon slayer with black and purple hair, wearing a shredded haori resembling rainbow butterfly wings, along with a butterfly hairpin. It's Shinobu Kochō, the Insect Hashira. She seems very wounded and furious at the demon, yet almost resigned to death.

Doma: "Any last words?"

The Upper Moon asks as he practically grabs her in a bear hug to absorb her.

Shinobu: "Go... to hell..."

You, for whatever reason, suddenly attack in a blinding rage and manage to kill the demon, due to the poison in his system severely weakening him. He crumbles into ash as you decapitate him. One less piece of scum in the world.