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, utilize 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 the Key to Time 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)
The Grace were later called upon by Abby and Zara, who in their deal with Kreekpolt had journeyed back to the planet of rain to ask them to heal his daughter Persephone. In the time since they had last met, the Grace now desired to maintain balance in the universe. They agreed to heal Persephone but only if her father gave his life for hers. The Grace would take the form of Daniel to converse with their children. They healed Abby and Zara, gifting them immortality and the power to go anywhere and any when. Before leaving they left a warning that even with eternal life the day would come when the sisters would choose to end their lives, the Grace having already seen this event. (AUDIO: The End)
The Grace appeared to Abby and Zara after they chose to die to prevent further damage to the vortex and established history after the result of their changes became apparent. The Grace would console them that even though they were unsuccessful at preventing the destruction of the sun they had died to save, the fact that they were willing to try had pleased them. The Grace then confessed to full knowledge of this outcome from the very beginning, but allowing them to go ahead so that they may observe them. In gratitude, they agreed to intervene, the Grace proving powerful enough to ensure the suns' survival. Their last act was to return Abby and Zara to the universe, stripping them of their powers and immortality to live a normal life in peace. (AUDIO: Consequences).
The Grace became inactive for a number of centuries after this event. Around 400 years later they would send Pool, their new agent, to seek out Abby and Zara, setting in motion the events that would return the Grace's powers to them. (AUDIO: The Bomb)
Pool would finally track down the sisters, begging for their assistance and bringing word from the Grace. Bringing them outside of time, to a realm inhabited by higher beings, Pool insisted the sisters enjoyed themselves and did their best to endear themselves to the other guests. The event having been organized by the Grace to deal with 'The shadow'. A darkness falling over the universe and preventing its observation by higher species. After much goading Pool revealed his true origins, being the embodiment of the Chaos Pool, the Grace in human form, the result of the Grace's previous fascination with the lives of the universe's inhabitants and the means by which they themselves could experience them.
The shadow was later revealed by Pool to be the consequence of Abby and Zara's innumerable changes to established history. Having gathered with other higher beings to make a decision. To give their lives in service of the sisters' cause and to maintain their powers; their latest efforts having expended much of the Grace's energies. The Grace had final say in the matter but could not decide on the right course of action, to ease the decision they personally met with and asked the incalculable peoples affected by the changes the sisters made. Still finding no clear answer The Grace were taken to meet a wise woman known to Abby and Zara during their mortal lives so that she may give him perspective. This woman being the old Zara. Having arrived at the critical moment of the past event The Grace finally saw the way clear, giving their lives freely to their daughters and in doing so being responsible for the return of their powers in the past. With this the Grace exhausted themselves totally. Disappearing forever. (AUDIO: The Dance)