Hur'q
The Hur'q (a Klingon term meaning "outsider") was the name given by the Klingons to a now-extinct species that flourished approximately a thousand years ago. They are most noted for invading the Klingon homeworld of Qo'noS, plundering whatever they could and destroying the rest. Amongst the most valuable treasures stolen by the Hur'q was the revered Sword of Kahless. The Hur'q appear to have originated from the Gamma Quadrant. (DS9: "The Sword of Kahless")
Though it was never stated, the Hur'q were most likely familiar with the Bajoran Wormhole long before its modern discovery. The Hur'q invasion was one of the first attacks that threatened the very existence of the Klingon Empire. It occurred some time before 2154 and was the last time the Empire was threatened on such a massive scale before the Klingon augment virus. (ENT: "Affliction")
In 2372, some ruins of their civilization were uncovered in the Gamma Quadrant by a Vulcan survey team. (DS9: "The Sword of Kahless")
A surviving Hur'q Hive ship was encountered in 2408 by the USS Rosenante after the Rosenante was captured by The Harvester.