Krop Tor

Krop Tor was a planet orbiting K37 Gem 5, a black hole. It takes its name from the Scripture of the Valtino phrase "Krop Tor" meaning The Bitter Pill, meaning that a Demon (K37 Gem 5) had attempted to swallow it, but finding it poison, had to spit it out. (DW: The Impossible Planet)

Myths and legends
Aside from the one giving the planet its name, many other myths and legends surrounded the place. The Ood seemed aware of it and linked it to a prophecy that "The Beast will rise from the pit". (TS: Tardisode 8, DW: The Impossible Planet / The Satan Pit)

Anomalies
Krop Tor presented itself as having a number of anomalies. It emanated a gravity funnel into space, preventing it from falling into the black hole. It also maintained stationary orbit by an unknown power source generating an inverted self-extrapolating reflex of 66 (46656) every six seconds. Even the Doctor could not provide a scientific explanation for the power source, which emanated from ten miles beneath the surface of the planet. (DW: The Impossible Planet)

Physical environment
Krop Tor had earthquakes and volcanoes. Though in stable orbit, it shook from the gravitational effects of the black hole. It was small, craggy and desolate world with little or no atmosphere. (DW: The Impossible Planet)

The Disciples and the Beast
A vanished civilization, The Disciples of the Light, had left traces of their existence on the planet, including a script left on pottery shards (TS: Tardisode 8, DW:The Impossible Planet).

Ten miles beneath the surface, the Doctor and Ida Scott from the Walker Expedition found a ruined underground city and a pit leading into darkness, which served as a prison for the Beast. (DW: The Satan Pit)