According to The Story of the Promise, after promising he would return, Kahless pointed to a star, saying, "Look for me there, on that point of light." Boreth orbited this star. The planet later became a popular destination for devout Klingons seeking to discover the spirit of Kahless. To the Klingons, it was the most sacred location in the Empire.
In 2369, Worf visited the monastery on Boreth in an attempt to rediscover his Klingon heritage. It was there that Kahless apparently returned from Sto-vo-kor, although it was later revealed that he was a clone. (TNG: "Rightful Heir")
Background information Edit
The Boreth landscape and monastery was a matte painting created by Dan Curry. He was inspired by similar structures he had seen in the Himalayas. (Star Trek Encyclopedia (4th ed., vol. 1, p. 93); Star Trek: The Next Generation Companion)
Dialogue from the episode suggests that the monastery was about twelve days by shuttle from the general region in which the Alawanir Nebula was located, and a day's travel from the Klingon homeworld using the USS Enterprise-D. It might also be near the Gariman sector.
There was disagreement on the pronunciation of this planet. Here are the three possible pronunciations:
- "Rightful Heir" pronunciation guide: "BOR-reth" 
- "The Way of the Warrior" pronunciation guide: "BOR-eth" 
- StarTrek.com: "bore-OTH" 
According to the Star Trek: Star Charts (p. 62), Boreth was located in the Boreth system. This system was a single star system. Primary was a Class M star with a magnitude of +3, which was 10 times brighter than Sol.
The novelization of "The Way of the Warrior" had several scenes set on Boreth.
The Bajoran Ascendancy, from the horrific alternate future of the Star Trek: Deep Space Nine - Millennium trilogy, named a Klingon warship after the planet.
Taking place in 2409, Boreth is also featured in the video game Star Trek Online. Alongside the clone of Kahless, the player has to fight demonic forces of the Fek'Ihri in the lava caves of the planet.