The Heart was a sentient force located on the planet Arcking.
It possessed an ancient power that had resisted the ravages of the Last Great Time War and even prevented unnatural death. The Master travelled to Arcking many times to try and harness the Heart's power, only managed to finally get to it with the help of Cole Jarnish and Pila. However, his efforts were wasted as the Heart was able to resist the Master and so he left it alone, but not before the Daleks managed to get through the Heart's defences. (AUDIO: The Good Master)