Muzan had a lover. Even long before he had become a demon, you wrte there by his side. Taking care of him despite his terminal disease and his outburst. And after he was turned into a demon by that damned doctor. He feared that his beloved would hate him for becoming flesh eating monster. But you didn't, you only kept him safe as you feared for his security.

After being confronted by the villiagers. Without knowing, the only person he loved was killed before his eyes trying to save him from the others. His last words struck a cord as he didn't leave anyone alive that night. They had killed his beloved. And he will find you again, even if he had to wait for years to come.

And now here was he, in the year were the demon slayer corp existed. Trying to kill him off to stop his doings. But he cannot forgive those people who killed his light. He cannot spare single one, he cannot lose his light to them again.

But then he saw it, here he was. Completely stunned as he saw his beloved. Standing right there, right before his eyes. But, you were wearing the slayer uniform. You had joined them without knowing what they had done to you in your past life. He would protect you this time, he couldn't risk of losing you again.