The Boreth Monastery is a monastery established by the Klingons. Shortly before his death, Kahless the Unforgettable pointed at a star in the skies over Qo'noS, and told the Klingon people that he would return there someday. The Klingons later came to Boreth, which orbited the star Kahless had pointed out, and established the Boreth Monastery. (TNG novel: Kahless; TNG episode: "Rightful Heir")
In the 23rd century the monks at Boreth were seen as the most devoted followers of Kahless. After Kol-Sha's attempted coup d'état, Ash Tyler took the child whom Voq and L'Rell had together to be raised by the monks at the monastery. (DSC episode: "Point of Light") The child was named Tenavik.
Another important responsibility of the Boreth monks was guarding and caring for the time crystals which grew on the planet. Guardianship of the time crystals was considered so vital that not even the Chancellor had any power on Boreth. (DSC episode: "Through the Valley of Shadows")
The clerics at Boreth, seeing the decay in Klingon society, decided to create a clone of Kahless, using genetic material that had been preserved over the centuries. They programmed this clone with memories from the original Kahless, and had him appear to Lieutenant Worf, who was attending a retreat on Boreth. Later, the deception was discovered. However, Worf felt that the clone had an important role to play in rebuilding Klingon society, and proposed naming the clone as Emperor. (TNG episode: "Rightful Heir")
In 2375, Boreth was the site of a battle between forces loyal to Chancellor Martok and those loyal to Gothmara. Martok confronted and killed Gothmara in the monastery. (DS9 - The Left Hand of Destiny novel: Book Two)
After Kahless disappeared and was replaced by a holographic representation, the Monastery was contacted to determine if he had returned there. Kahless had not been seen by the staff, who revealed that all records of Kahless's cloning had been destroyed in the assault four years earlier and no backups had been made. (TNG novel: A Time for War, A Time for Peace)