Chapter 1: the magic box

Timeline: during kwamis choice: Both tikki and Plagg are flying through the air and feels a energy surge, they feel the urge to find it and put their energy into it, after they put their energy into it (which is a multiversal portal that lead to the kny world during the final selection), they merge to create the miracle box and grimoire

During the hand demon fight Tanjiro finds the miracle and grim and first transforms with the miraculous of the black cat