Disclaimer; I do not own demon slayer or any of the anime that is mentioned
Giyuu(p.o.v.)
Muzan Kibutsuji !!! I heard Tanjiro scream out as he attached the demon king. I, who just lost my right arm, grabbed my sword with my left hand, charged in, determine to pin Muzan in place, regardless if I have to die with him. As I approached, I noticed that the sun is just on the horizon. I also noticed that both Muzan and Tanjiro are on the ground, and that Muzan was trying to do something to Tanjiro.
I moved faster than I ever thought possible, blurring from where I was, and appearing right next to Tanjiro and pushing him out of the way, and shocking Muzan, as he plunged his fingers into my chest. I pushed through the pain and stabbed my blade through his chest and into the floor, pinning him there. I then felt excruciating pain all over my body.
I pushed through the pain and held him there with all my strength as the sun rose higher and higher, I looked back to where Tanjiro and the others are, seeing most of my comrades in very painful states right now. Nezuko, Tanjiro's sister, injected what I assumed was the cure, allowing him to sit up, look around in a panic, his eyes stopping and locking on to mine as he whispered my name, Giyuu-san, so quietly that I had to read his lips to understand what he said.
The sun is just high enough for the rays to touch my back. I started to feel a burning sensation on the uncovered parts of my body. I then realized that Muzan, in a desperate attempt to survive was infusing his blood in me, no not me, it was meant for Tanjiro. I just got in the way. At least this way he'd actually die.
Tanjiro is stronger than me, in the sense that he'd probably conquer the sun if he were to become a demon, like how Nezuko. And judging from Muzan's face, that was exactly what he was hoping for, I looked at the terrified expression on his face, and said,"
End of the line Muzan. Your reign of terror ends right here and right now". He screams in pain as the sunlight disintegrate him until there's nothing else left of the demon king. I stood up and felt the sun on my face for the last time. I looked over to Tanjiro and saw that he and his friends had survived, him with his arm outstretched, reaching out to me.
Tears streaked down his and everyone's faces, they crying for me, I realize. I smile at them one last time, and said,"Live a good and happy life. Get married, have kids and protect your family. Good bye, everyone, good bye, brother, take care of your sister for me." It was the last thing I could say as the rays of the sun consumed me.
I heard Tanjiro bawl one last time," Big brother Giyuu!!!" One last time last time before darkness engulfs my vision.
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3rd (p.o.v.) uknown place
As Giyuu floated unconsciously in a void like space with colors merging into one another, a being of unimaginable power took notice of him. "Hmmm, he not only managed to survive, he also absorbed the chaotic energy of this place, impressive." It said, impressed. It looked through his memories, and decided to take him to a place to talk to him about something, and maybe give him some help.