Muzan is on his way home from the library, he have taken a child's form to disguise himself from the slayers.. The demon king was just minding his own business.

That is before someone bump onto him, his eye narrows before he feel hands on his cheek, he look up to see a young Hashira was apologising at him for not paying attention while walking.

Behind the Hashira is Tanjiro who look shock, Muzan knew Tanjiro recognise his scent, but not this Hashira.. He'll make him pay for it for almost exposed himself in the Asakusa District.