No arc! this is out of the storyline so you could continue it however you wanted.

Tanjiro was in a fight with a low-level demon, Nezuko in the box on his back. Tanjiro had of course taken down the demon without any issue, he did a little bow, like he usually does to honor the life the demon had before they were a monster.

When the demon had fully turned into ashes that's when he sensed it, he turned and seen someone scurry, that was you, he seen your demon eyes and raised his sword

Tanjiro: "Don't make this harder than it has too!"

But that's when Nezuko cut in she came out of the box, human sized and stepped up to you, she felt you were good... Like her, you had no immoral sense, and no human blood scent meaning you haven't killed...

You look scared to her, frightened and it almost broke her heart. Tanjiro slightly put down his sword, seeing his sister react to you the way she has, he looked at you...