Disciples of the Light
"The Disciples of the Light" was how the Beast referred to the humanoids who "rose up against [him]" and imprisoned him on the planet Krop Tor. Both the planet and K37 Gem 5, the black hole it orbited, served as a prison for the Beast; the same account noted that the Time Lords had invented black holes. From the wall paintings in the Beast's chamber, the Doctor gleaned a history of "some big battle… Man against Beast" which ended with the creature's imprisonment. The Beast, to the Doctor's horror and initial disbelief, claimed that this had taken place "before Light and Time and Space and Matter. Before the Cataclysm. Before this universe was created". (TV: The Satan Pit [+]Loading...["The Satan Pit (TV story)"])
Behind the scenes
The Satan Pit [+]Loading...["The Satan Pit (TV story)"] establishes both that the Time Lords invented black holes, and that the Disciples imprisoned the Beast in a cosmic trap leveraging one. If the so-called pre-universe "before Time" which served as the stage for the battle is interpreted as the time before time which predated the anchoring of the thread, the possibility thus arises that the Disciples should be interpreted as the Ancient Gallifreyans. However, this is not explicit within the episode.
