The planet Khitomer
The Khitomer Massacre was a Romulan attack on the Klingon colony on Khitomer in 2346.
The attacking ships, presumed to be Romulan Warbirds, disabled the planetary defence and initiated a massive orbital bombardment resulting in widescale destruction of the population center and 78% estimated casualties. (TNG: "Sins of the Father")
Nearly all of the 4,000 Klingon colonists there were killed. The attack was engineered by the Klingon traitor Ja'rod, while the Romulan Star Empire was still supposed to be "an ally" of the Klingon Empire. (TNG: "The Neutral Zone", "Sins of the Father") Two years earlier, Romulan forces had already attacked the Klingon outpost on Narendra III. (TNG: "Yesterday's Enterprise")
Mogh, father of Worf, was killed in the massacre but his son was among the colony's few survivors. Worf was rescued by Sergey Rozhenko of the Federation starship USS Intrepid, which was the first ship to respond to the colony's distress call. Worf never forgot the massacre, and hated Romulans for most of his life. (TNG: "Heart of Glory", "Family", "The Enemy")
A small number of Klingons, including L'Kor and Gi'ral, were captured by the Romulans after the attack. They were interrogated for three months, following which the Romulans attempted to exchange the prisoners for territorial concessions. The Klingon High Council refused to accept that Klingons would allow themselves to be captured, and would not negotiate. The Romulans would have executed the prisoners, but Tokath argued successfully that they should be allowed to live. The Romulans placed the Klingons at a remote camp on Carraya IV, under Tokath's charge; there the prisoners and their guards remained, raising families together, forgotten by the outside world until they were discovered by Worf in 2369. Several of the Klingons' children, including Toq, left Carraya with Worf, although to preserve their family honor Worf did not disclose their true origins. (TNG: "Birthright, Part I", "Birthright, Part II")
From 2366 to 2368, Mogh was generally considered among Klingons to have collaborated with the Romulans to enable their attack on Khitomer. In fact, it was the traitor Ja'rod, who gave the colony's defense shield codes to the Romulans, which was, however, covered up by his son, Duras. In 2366, Worf tried to challenge the accusation, but in vain: although Chancellor K'mpec knew that Ja'rod was the traitor, he allowed the lie to stand in order to protect peace within the Klingon Empire. Worf dropped the challenge and was discommendated by the Klingon High Council. Only Worf, Duras, K'mpec, Captain Picard and Kurn (Worf's younger brother) knew the truth at this time. (TNG: "Sins of the Father")
After the Klingon Civil War broke out late in 2367, Chancellor Gowron permitted the truth to be revealed, and restored the honor of the House of Mogh. (TNG: "Redemption", "Redemption II")
A similar massacre is memorialized by an obelisk (VOY Memoria), during which episode Capt. Janeway references Khitomer.