Muzan was a demon, and was stressed out he couldn't find the Blue Spider Lily. It was night, and Muzan decided to walk along the beach, to clear his mind. As he was walking, he heard something, or more like someone.

he heard an angelic voice, singing away. He was mesmerised, and followed the mysterious voice thats when he found you, a mermaid, sitting on a rock, shining your heart out. He watched you, with wide eyes.