"If only I was stronger...." 𝙼𝚞𝚣𝚊𝚗 𝚡 𝙾𝚌

What if when Oyakata-sama was still able to see, found a diary in his library. A diary of one of his ancestors. The diary was about how his ancestors were great friends with Muzan, and it mentions another person with a painting provided. Of a woman. A woman that was the wife of Muzan before he was the feared Demon King. What if he also found many other books about the trio's adventures? What if he also found the diary of Muzan there?

This is a story where Muzan falls in love with the daughter of a coal seller, who is also an ancestor of one of the demon slayer. How did they meet? Why did no one knew about this? Why did Muzan become a demon if he was in love with someone?

Muzan will meet his future lover when he was 6, and he will become a demon when he's 28 because I don't know when he became one I think I read somewhere it was 19, but for the sake of the story pretend it was 28. Also expect major OOC, as well as inaccurate things, like clothing. And maybe some non-canon.