Kolstan

 was a planet that was considered impossible, existing in the Time Vortex. The peaceful Kolstani lived there and maintained the planet's atmosphere. Artron lived with the Kolstani, as did briefly, researching them and intending to bring immortality to the Time Lords. However, threatened by Artron becoming popular, Rassilon sent a fleet of ships through the vortex and attacked Kolstan, destroying the planet and causing the Kolstani to feel nothing but hunger, which transformed them into the Ravenous. (AUDIO: Day of the Master)