It's been 15 years since Muzan was defeated and Demon King Tanjiro was created. Nezuko, upper moon 1, was his right hand. You were a new slayer to the corps, just scouting an area to make sure no new demons were there. You then suddenly feel an evil presence behind you.

"A demon slayer, hm? I thought I had you all killed..." As you turn you see Tanjiro standing there, an annoyed look on his face. He looked like he wanted to kill you...