Romulan Star Empire
Khitomer was an inhabited planet located in the Khitomer system. Khitomer played a vital role in the development of the Klingon-Federation alliance, being the site of the Khitomer Conference and the victim of a Romulan sneak attack on this planet's colony, which, in turn, emphasized the importance of the Klingon-Federation friendship. (TNG: "The Neutral Zone", "Sins of the Father")
In 2293, Khitomer was considered to be a neutral site between the Federation and the Klingon Empire, and was located near the Romulan border. In that year, the Khitomer Conference was held at Camp Khitomer on the planet. The conference started the first full peace negotiations between the United Federation of Planets and the Klingon Empire and gave birth to the Khitomer Accords. Originally, Earth had been selected as a venue, but after Klingon chancellor Gorkon was assassinated on his way to Earth, the conference had to be relocated to Camp Khitomer. (Star Trek VI: The Undiscovered Country)
By 2346, a Klingon colony existed on Khitomer. On stardate 23859.7, the colony fell victim to a Romulan attack. In the attack - which later became known as the Khitomer Massacre - 4,000 colonists were killed, amongst them Mogh and Ja'rod (TNG: "Heart of Glory", "Sins of the Father"). Only two survivors were believed to have been rescued by the USS Intrepid; however, a group of colonists captured by the Romulans were allowed to live and were taken to a prison camp in the Carraya sector, remaining undiscovered until 2369. (TNG: "Birthright, Part I", "Birthright, Part II") An obelisk was erected on Khitomer as a memorial to those killed. (VOY: "Memorial")
According to the Star Trek: Star Charts (pg. 56), the Khitomer system was located in or near Klingon space, in the Beta Quadrant. This system was a binary star system. Primary was a Class F star with a magnitude of +3, which was ten times brighter than Sol. Secondary was a Class F star. Tertiary was a Class K star.
The events surrounding the Khitomer Massacre were documented in the The Lost Era novel, The Art of the Impossible.