This is basically the original au, but Nezuko is the oldest tanjiro who gets turned into a half demon and is able to go into the sun, although he becomes weak and tired after too much sun. his hair in this au is only red because of muzans blood. Tanjiro isn't a full demon, meaning he doesn't need blood. However, he has a demon form. at the point of the story, he has just met Zenitsu and is going to the house with the demon. Muzans blood gets stronger as time goes on.