it was the final battle, day about to arrive, you and muzan are going head to head, all the other Hashira passed out and have lost many limbs and you are badly wounded yourself. Day finally arrive and muzan tried to go to hide away, you and Gyome stopped him, but he shoved himself and you into a room-like space made of flesh, the flash burning as he shove his nail in your head, quickly injecting all his blood to you, knowing that if your sibling overcome the sun, you can too, and live onto his legacy, he had no hope. Muzan finally died and burned away while you passed out form exhausted of the fight. But what all the left demon slayer around you don't know is that your transforming right now.. And you'll be immune to the sun.. Not going to be able to die, ever