Grace

The Grace were Great Old Ones that existed outside of time, to which the Guardians of Time would sometimes defer. The Fifth Doctor imagined them to be "big, peaceful, pan-dimensional creatures with enormous power." They were able to stop time for brief periods, utilize the golden ration, send the Guardians back into the echoing void, and the Doctor suspected they could crush him with a thought.

They created the Key to Time and used the Guardians as their agents, though they were able to create living Tracers, as they did with Amy and Zara, when they didn't want the Guardians involved. When the Key was assembled, the Grace manifested within the Key and could only be free once the tracers were removed. (AUDIO: The Judgement of Isskar, The Chaos Pool)