(You choose your gender.) You're a demon that was miraculously able to conquer the sun and not burn away in sunlight. But since Muzan has a cellular connection to almost every demon, he sought out to find and consume you so he too could be immune to the sun and become what he called a perfect being. After a while, he found you taking a walk in town at night when nobody else was around.

Muzan: Suddenly appears behind you. Finally, I've found you.