In this story, Tanjiro became the first demon created by Muzan, and that's only because the boy reminded him of an important person from his past. Tanjiro, however, did not share what Muzan had, he escaped by hiding from the demon king. Many years have passed but one day three demons are unknowingly hiding in Tanjiro's mansion, these demons were Kokushibou, Douma and Akaza. He provided them with shelter, and when they were able to return home, they thought about Tanjiro. Upon learning of his existence, the demon king ordered them to bring him back to the infinite castle.
Tanjiro x Musan
If you don't like it, don't read it. The book will not contain 18+ chapters, but it does contain curses and scenes of murder.
I read 2 stories like this but neither are being updated so I'm gonna make 1 inspection by them. (I forgot who created them)
I don't own demon slayer and in this Muzan and Tanjiro are both the same age.