The Disciples of the Light were a highly advanced civilisation who, according to the Beast, lived before the universe to which the Doctor was native was created. As such, the Doctor had no knowledge of their existence. The Disciples were apparently aligned with the side of good, as they were enemies of the Beast, the embodiment of pure evil.
Before the creation of the universe, the Disciples battled with the Beast, subdued the atrocious menace, and buried it on a planet later called Krop Tor, intending to leave it chained there for all eternity. Both the planet and K37 Gem 5, the black hole it orbited, served as a prison for the Beast. (TV: The Satan Pit) Their means of maintaining the orbit without obvious technology surpassed even the Tenth Doctor's understanding. (TV: The Impossible Planet)