Shared headspace (mental world) occupied by Wakaba Mutsumi and her dissociative alternate identity Mortis in BanG Dream! Ave Mujica.
This space is usually depicted with the motifs/settings of a room with a wall of CRT Television screens which show memories shared between the two and/or a theater whose audience is filled with Clones of Mutsumi or Mortis, as well as the presence of their doll personas.