This planet was located "at least" 1,000 light years from the Sol system. Its existence remained unknown well into the mid-23rd century. Up to that point the planet had not appeared on contemporary Federation star charts. This was due to active concealment of the fact of the existence of the planet by its inhabitants.
This technologically advanced species wanted to help Earth survive its early technological age. To this end, around 3000 BC, they took certain Humans to serve as the forebears of a pool of genetically-perfect Earth operatives. As Gary Seven, one of their Class 1 supervisor, said on Earth in 1968: "A truly advanced planet wouldn't use force. They wouldn't come here in alien forms. The best of all possible methods would be to take Human beings to their world [and] train them for generations until they're needed here."
These agents were Human but flawless in design: not even the Vulcan nerve pinch could stop them. They were transported back to Earth as needed by means of an incredibly powerful transporter beam which, in the one case where it hit the Enterprise, shook the vessel violently and locked her circuits wide open (fusing those having to do with recording). At least one agent was accompanied on the interstellar beaming by a humanoid who preferred to appear to be a domestic cat (or vice versa).
Gary Seven followed two agents dispatched to Earth, 201 and 347, on the day in 1968 when there occurred a major assassination, an Asian coup d'etat, and the launch into orbit of a US nuclear weapons platform. Seven came because the agents, dead in an automobile accident, had been incommunicado. He took over their assignment – which was to effect a misfire and near-holocaust to frighten the USA and the other major powers out of their foolish "balance of power" strategy – and completed it by the skin of his teeth. (TOS: "Assignment: Earth")
According to The Rise and Fall of Khan Noonien Singh, Volume One, the Assigners' planet was a cloaked planet of the Aegis (β) in system Zeta Gamma 537 (β), and was 50,000 light years from Earth.
In the Star Trek: Department of Temporal Investigations novel Watching the Clock, the planet was described as having orange skies, silver trees, and cities made of crystal spires. It was home to "vast, extraordinary beings, with...great eyes."