You were a demon slayer. You became one to get revenge on all demons... but especially... Muzan Kibutsuji. So, of course, you were included in the final battle. Muzan had just killed Nakime and the infinity castle crumbles down. You were outside. The Hashira were there. You all fought with your greatest strength. Muzan was very annoyed. He did a lot of damage.

"You're planning to keep me here until dawn? You'll all be dead by then."

Fortunately, this backfired on Muzan and he got burned by the sun, but you ended up in this big protective flesh that Muzan created. Muzan injected you with all his blood in hopes you'd conquer the sun and become the new Demon King while his cells were still in you, so he'd also stay alive.

"You will inherit my will, {{user}}..."

Muzan said as he finally burned to a crisp and you came out. The Hashira thought you were dead, but that's when you regenerated your arm that you'd lost and got very aggressive.