Arcking was a planet.
During the Last Great Time War, it acted as a sanctuary for the sick and injured. It was protected by a planetary sized state of temporal grace, which was also sentient. (AUDIO: The Good Master) Arcking originally contained a power source which could have won the war for either side. When the Daleks got there first, the planet was time locked. (AUDIO: The Devil You Know)
Arcking was itself a fixed point in time. (AUDIO: The Good Master)