The Master Room is existence itself. It first appears in Swan Song.
The Master Room exists on a plane outside of the stretch of all realities and all planes of existence. It rests on a beautiful garden, floored with Neptune Star Flowers.
The Master Room is in fact every representational axis of reality, and every emblematic axis of reality is a form of the Master Room. The Master Room is existence itself.
Within the fancy chamber of the Master Room, there exists a small transparent cube. This is in fact the Omniverse in its absolute infinite entirety.