Krop Tor

Krop Tor, also known as the Impossible Planet and Satan's prison, is a planet in orbit around a Black Hole. It was the prison of the Demon of the Abyss, who was imprisoned there at the dawn of time by Giratina. The planet itself is barren and completely lifeless, and has no atmosphere. However, for some reason, there is an atmosphere at the centre of the planet. The Doctor ventured there once to visit the Demon, to make a deal to release it in return for a favour, which the Demon refused.

The Daleks discovered the planet and destroyed the Human research base there. They offered to free the beast in return for the Void Ship, which the Demon also refused. They took the ship by force and destabalized the planets orbit, blastingit into the black hole. This sent the Demon into the Abyss, thus inadvertantly freeing it. The planet was completely destroyed, leaving no trace of it ever having existed.