|Physical and technical specifications|
|Usage and history|
- "Just something I was playing with. I'm bored of it now."
- ―The Rival to Ellen
- The Rival: "This is your world now."
- The Captive: "Except you make the rules. I'm in your possession, right?"
- ―The Rival and the Captive[src]
To outward appearances, this artifact resembled a green marble, small enough to fit in an adult's palm. It served as a gateway to an abstract dimension, known as the White Room, of apparently infinite space. This prison was capable of holding at least one person, as a prison of the mind from which there was apparently no escape.
Outsiders were able to gain access to the prison. The Rival spoke to his captive through the walls of the White Room as a disembodied voice, and incarnated himself inside to confront the Captive directly. The outside world affected the interior of the marble in certain ways; when the Rival discarded the marble underneath a chest of drawers, the White Room became dark, fading to the point of pitch.