Ancient invaders who plundered the Klingon homeworld a thousand years ago and carried off the Sword of Kahless.
The Hur'q are an alien species who invaded Qo'noS roughly a thousand years before the 24th century. They looted the Klingon homeworld, and among the treasures they took was the Sword of Kahless, the bat'leth said to have been forged by Kahless the Unforgettable himself.
The Klingons have never forgotten the Hur'q, and their memory survives mostly as legend. Kor enlisted Worf and Jadzia Dax to track the sword to a world in the Gamma Quadrant, but the relic proved so divisive that Worf and Kor sent it adrift into space, in The Sword of Kahless.