According to The Story of the Promise, upon departing for Sto-vo-kor, Kahless promised he would return one day and pointed to a star: "Look for me there, on that point of light." Once the Klingons developed spaceflight, the Followers of Kahless would travel to the star system and build a monastery on Boreth, a planet orbiting the star. (TNG: "Rightful Heir")
The monastery, the only structure on the planet, was built near natural lava caves, and also served to protect a rare mineral native to the planet: time crystals. As of the mid-23rd century, Klingon lives had guarded these crystals for generations. (DIS: "Through the Valley of Shadows")
In 2257, 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. (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 with a time crystal. (DIS: "Through the Valley of Shadows")
The script for "Rightful Heir" describes Boreth as "mountainous... desolate... no cities or dwellings of any kind except for a single temple situated high up on the tallest mountaintop."
The pronunciation of "Boreth" has been explained by different sources as:
- "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 near the Gariman sector. The system's primary was an M-class star. In "Through the Valley of Shadows", a star chart otherwise modeled off Star Trek: Star Charts showed Boreth to instead be near K't'inga and Halee.
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.