Apocalypse Clock (The City and the Clock)

 was a legendary artefact which could hold back the end of the universe. The Doctor sent Bernice Summerfield to find it. It was located on Merin. The ghosts on the planet wanted to use the clock to resurrect themselves. The Doctor thought that it could be used to create an entropy shield. (AUDIO: The City and the Clock) After the Doctor worked out the equations, The Master used it to summon back the Great Old Ones. The Doctor used the energy that this created. (AUDIO: The True Saviour of the Universe)