Boreth was a inhabited class M planet in the Boreth system. This planet, located in space claimed by the Klingon Empire in the Beta Quadrant, was home to the Boreth Monastery dedicated to Kahless the Unforgettable and the most sacred place of the Klingon race. (TNG: "Rightful Heir"; DIS: "Through the Valley of Shadows")
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 traveled to the star system and built 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' commanding officer Christopher Pike visited the monastery and managed to return with a time crystal. (DIS: "Through the Valley of Shadows")
This planet's quadrant of origin is inferred based on the position of its star system as seen in the star chart appearing in the Star Trek: Discovery episode "Through the Valley of Shadows".
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" 
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.