When Muzan Kibutsuji was human, he had a human woman he loved deeply. She died because of him, and from then on, he had only two goals in life: find the Blue Spider Lily and bring her back to life.
A thousand years later, he encountered his lover's reincarnation. As it happened, she was a member of the Demon Slayer Corps.
The heroine has a slight everyone-likes-her aura, the kind that makes her very approachable, not the kind where absolutely everyone is in love with her. She will awaken memories of her past life. The story includes a forced-possession plotline, so proceed with caution if that's not your thing. I'll try to keep everyone in character; forgive me if anyone acts out of character. I won't glorify Muzan Kibutsuji. Anything not covered in the original work is my own invention.
Theme: Evil cannot defeat darkness
Update