Apocalypse Clock (Unbound Universe)

The Apocalypse 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 that would protect a particular part of the universe from the imminent collapse of reality that would be caused in the aftermath of the War. (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 to both power the shield around the safe zone that he had been working to create and boost the basic time machine he was using to give it the required power boost to take Benny back to her home universe, although this left him trapped in Benny's reality. (AUDIO: The True Saviour of the Universe)