The High Clerics were a group on Boreth.
The story of how the Sword of Kahless was forged was never written down and only known to the High Clerics, so that if Kahless returned, they could be sure it was Kahless. The knowledge was put to use when Kahless appeared to have returned in 2369. (TNG: "Rightful Heir")
Background information Edit
In DS9: "The Way of the Warrior" Worf spoke of his conversations with the clerics of Boreth, while in "Rightful Heir" only the word "High Cleric" was used. Worf might have used cleric as shorthand, or clerics might be a lower rank. Similarly, in "The Sword of Kahless", Kor complained that the clerics of Boreth had cooked up a Kahless clone in a vat.