Grace

The Grace were Great Old Ones that existed in a state of non-existence outside of time and space, who created the Key to Time and used the Black and White Guardians as their agents.

They worked to hold the universe together and occasionally stepped into reality to prevent it from falling apart. Zara saw them as having the appearance of imps, and stated they had no bones. (AUDIO: The Prisoner's Dilemma) Abby stated they were too big to squeeze into ordinary time and space. (AUDIO: The Flood)

The Fifth Doctor imagined them to be "big, peaceful, pan-dimensional creatures with enormous power" since they were able to stop time for brief periods, utilise the golden ratio, and send the Guardians back into the echoing void. The Doctor also suspected they could crush him with a thought. (AUDIO: The Judgement of Isskar, The Chaos Pool)

Though they used the Guardians as agents, they were able to create living tracers, as they did with Amy and Zara, when they didn't want the Guardians involved; Amy accompanied the Fifth Doctor to assemble to Key while Zara pursued it for herself. When the Key was assembled, the Grace manifested within it and could only be free once the tracers were removed. However, the Doctor dispersed the Key within the Chaos Pool in an attempt to destroy it, so the Grace may have been returned outside the universe. (AUDIO: The Judgement of Isskar, The Chaos Pool)