The Romulans used Klingon codes to destroy the outpost
Worf's father Mogh was initially charged with being a traitor. He and Worf were eventually cleared
The Khitomer Massacre was a Romulan attack on the Klingon colony on Khitomer in 2346.
Nearly all of the 4,000 Klingon colonists there were killed. The attack was engineered by the Klingon traitor Ja'rod. The ambush was part of a larger Romulan offensive against the Klingon Empire, which began with the attack on Narendra III in 2344.
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 Intrepid, which was one of the first ships to respond to the colony's distress call. Worf never forgot the massacre, and hated Romulans for most of his life.
From 2366 to 2368, Mogh was generally considered by Klingons to be the traitor that enabled the Romulan attack on Khitomer. In fact, it was the traitor Ja'rod, who gave the colony's defense codes to the enemy. This fact was carefully covered up by Chancellor K'mpec and the Klingon High Council. Ja'rod's son, Duras, had become a powerful political figure in the Empire. To release the information of his father's betrayal would have plunged the Klingon Empire into civil war. This secret was threatened with exposure when Worf, son of Mogh, challenged the findings of the Council. However, in order to preserve the peace, Worf dropped the challenge and was discommendated by the Klingon High Council. Only Worf, Duras, K'mpec and Captain Picard knew the truth at this time.
After civil war broke out in late 2367, Chancellor Gowron permitted the truth to be revealed, and restored the honor of the House of Mogh.