Shen'ihr
While humans know little of the Shen'ihr, the closest neighbouring Elder Race to the Grey, other Younger Races that are geographically closer to them and that have particularly long memories do remember more of them. They are the newest race to take on the mantle of an Elder Race, and some Younger Races have been around for long enough to be aware of this -- or to remember when the Shen'ihr replaced their prior patron as the Elder Race charged with their care. Those that are aware of it know that the Shen'ihr were once a Younger Race, discovered and taken under the wing of a prior Elder Race, whose name has been viciously and thoroughly eradicated from records in the aftermath of the Shen'ihr destroying them. Surviving records only make reference to 'the Dark Ones' or 'the Enslavers', the latter of which is how they are generally referred to by historians.
The Enslavers were in the habit of doing literally that -- taking Younger Races as slaves, and using Potential powers to control them, taking away their free will and ability to develop. They kept their practices hidden from the other Elder Races, putting on a good front, and as it is generally the practice of the Elder Races to let each other look after their own business, there were few if any signs for the other Elder Races to see. However, the Shen'ihr had a particular gift for resisting the practices of the Enslavers, as well as for patience, being a remarkably long-lived race for a Younger Race, which are typically not yet advanced enough to have developed an extended longevity. Through the use of their racial gifts, they organized, planned, and executed an incredibly effective revolt, freeing the Younger Races from the enslavement of the Elder Race, and hunting down and destroying all of the Enslavers they could locate. They also secured sufficient proof of the Enslavers' activities that when the Elder Races began to coalesce to move against them, they were able to prove the justice of their cause, and the Elder Races opted to not destroy the Shen'ihr for their temerity in committing genocide against one of their number, but instead elevate them to the status of an Elder Race, where they could use their sense of justice to guide those that the Enslavers had previously had power over to a brighter future.