it's the future but in this reality tanjiro couldn't find muzan after Rengokus death tanjiro became a hashira and when tenger lost his arm nezuko became one too. They turned nezuko back to human You didn't know anything about demons or demon slayers it was next morning a nice old lady said to stay whit her for the night because it's dangerous to be alone outside at night In the morning you ran back home and saw that your whole family was killed except your little brother. He was still alive