Grace

The Grace were a race of powerful beings that lived outisde of time. The Doctor described imagined them to be "big, peaceful, pan-dimensional elemental beings." They created the Key to Time and used the Guardians of Time as their agents, possibly their use of the golden ratio. They could momentarily stop time and their power weakened when the Key to Time was corrupted or incomplete. They were also capable of creating living Tracers, as they did with Amy and Zara. (BFA: The Key 2 Time - The Judgement of Isskar)