An invasion of the Klingon homeworld in the 14th century by a Gamma Quadrant people the Klingons called the Hur'q, who looted its treasures, including the revered Sword of Kahless.
The Hur'q invasion was a formative humiliation in Klingon history. Around a thousand years before the 24th century a species from the Gamma Quadrant descended on Qo'noS and carried off many of its cultural treasures. The Klingons named them Hur'q, a word meaning outsider.
The most grievous loss was the Sword of Kahless, the first bat'leth, forged by the founder of the Empire. In the Deep Space Nine episode The Sword of Kahless, Kor, Worf and Jadzia Dax trace the blade to a Hur'q vault in the Gamma Quadrant in 2372.
The invasion left the Klingons with a lasting suspicion of outsiders and a determination never again to be caught defenceless on their own world.