Boreth was a class M planet located in the Klingon Empire, and the most sacred place of the Klingon people. The only non-native structure located on the planet was a monastery dedicated to Kahless, the most sacred site in the Klingon Empire. It was positioned near several natural lava caves. (TNG: "Rightful Heir"; DIS: "Through the Valley of Shadows")
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 circled this star, and the most devout Followers of Kahless awaited his return there. As such, the monastery was occupied by guardians, High Clerics, clerics, and monks. (TNG: "Rightful Heir"; DS9: "The Way of the Warrior", "The Sword of Kahless"; DIS: "Point of Light")
However, the monastery on Boreth was not only built to house the Followers of Kahless, but also to protect a rare mineral native to the planet, time crystals. Those protecting the crystals called themselves the timekeepers. Even the Klingon chancellor did not hold sway over the monastery or the monks who guarded the crystals. As of the mid-23rd century, Klingon lives had guarded these crystals for generations. (DIS: "Through the Valley of Shadows")
During the 23rd century, it was said that no one left the monastery, and no one who did not belong there ever got in. Contact with outsiders had been almost nonexistent ever since the world was still young. (DIS: "Point of Light", "Through the Valley of Shadows")
In the late 2250s, the Section 31 ship NCIA-93 delivered the infant child of the Chancellor of the Klingon High Council L'Rell with Voq to the planet, to be placed in the care of its monks. Considering the child to be a vulnerability, L'Rell initially hoped to keep the child a secret from her enemies within the High Council. However, following an attempt on the boy's life, she publicly proclaimed the child had been murdered, when in reality, she secretly sent him to be raised on Boreth as a monk and as a son of none. (DIS: "Point of Light", "Through the Valley of Shadows")
Later, in 2257, a red burst appeared over the planet. The USS Discovery managed to negotiate passage to the planet from Chancellor L'Rell. After using a spore drive jump to reach the planet, Discovery's commanding officer Christopher Pike visited the monastery and managed to return a time crystal. (DIS: "Through the Valley of Shadows")
In 2369, the high clerics of Boreth produced a clone of Kahless and orchestrated his apparent return from Sto-vo-kor in order to restore honor to the Empire. Klingon Starfleet officer Worf, who had been put on leave and had chosen to go to Boreth in order to seek spiritual enlightenment, was present at the monastery when this happened. With the help of Starfleet, the nature of the clone was eventually revealed. (TNG: "Rightful Heir")
Following the destruction of the USS Enterprise-D in 2371, Worf took extended leave from Starfleet to visit Boreth again. He found his discussions with the clerics there to be most enlightening. As his leave neared its end, he was pulled back from his studies due to the Federation-Klingon War, but he hoped to return afterward, even considering resigning from Starfleet. Eventually, Worf remained in Starfleet. (DS9: "The Way of the Warrior")
Background information Edit
In "Rightful Heir", 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)
Although the final shot of the matte painting contained clouds and might not have shown the entire complex, the monastery seen in "Through the Valley of Shadows" appeared to be a completely different structure. Given that in that episode it was said to be the only non-native structure on the planet, it might have been rebuilt at some point.
Dialogue from "Rightful Heir" suggests that the monastery was about twelve days by shuttle from the general region in which the Alwanir 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 Star Trek: Star Charts (p. 62) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Boreth was located in the Boreth system. The system's primary was an M-class star.
The novelization of "The Way of the Warrior" had several scenes set on Boreth.
The Bajoran Ascendancy, from a horrific alternate future in 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.