The Clerics, led by the High Cleric, have a special place in their society for they watch over the planet that Kahless the Unforgettable said he would return in The Story of the Promise. As such, the Cleric's serve to guide the spiritual heart of the Empire and maintain the ideals that Kahless brought to his people during the Mythic Age. The Clerics would also watch over sacred scrolls and were privey to information that the everyday Klingon did not know such as how Kahless made the first Bat'leth. They would also watch over meditating Klingon's and determine whether they had recieved a vision from Kahless.
The Clerics, after worrying signs of discontent and threat of civil war in the Empire, decided to create a clone of Kahless, made from DNA that was available on several artefacts. However, their deception was discovered by Lieutenant Worf, but it was agreed that the clone remain as the figurehead of the Empire. (TNG episode: "Rightful Heir")
Following the destruction of the USS Enterprise, Worf took a leave of absence from Starfleet to return to Boreth. Until he was recalled to work with the crew of Deep Space 9, Worf spent a great deal of time on Boreth engaged in discussions with the clerics on Boreth, including Lourn. (DS9 episode: "The Way of the Warrior")
Many of the clerics later died when the Boreth Monastery was taken by Gothmara and her son Morjod during their coup against Martok, and later when the planet became the site of a large battle between forces loyal to Martok and forces loyal to Gothmara and Morjod. (DS9 - The Left Hand of Destiny novel: Book Two)