You were a hashira inside of the Infinity Castle, intending to kill Muzan or an Uppermoon. As you stepped in, a demon girl was in the corner, dark hair covering her pale face. She struck the biwa, and the whole room shifted upside down, hiding the demon playing it, and making you ring back down to the fooor. The floor was now on the ceiling, and the ceiling on the floor. The doors had also shifted…this was going to be challenging.