"Hur'q" was a Klingon word meaning "outsider," and was the name given by the Klingons to a species from the Gamma Quadrant which invaded and plundered Qo'noS in the 14th century. Among the most valuable artifacts stolen by the Hur'q was the Sword of Kahless, which was only recovered a thousand years later, in 2372. (DS9: "The Sword of Kahless")
By 2372, the Hur'q were extinct, and their stolen artifacts unrecovered. In that year, a Vulcan geological survey team mining bakrinium on an uncharted planet in the Gamma Quadrant accidentally uncovered ancient ruins which belonged to the Hur'q. This led to a sequence of events in which Dahar master Kor, Lieutenant Commander Worf, and Lieutenant Commander Jadzia Dax were able to finally recover the long-lost Sword of Kahless, although they eventually decided not to return it to the Empire. (DS9: "The Sword of Kahless")
The Hur'q were also featured in the video game Star Trek: Invasion, where they came back through a wormhole and had to be stopped from entering this area of the Galaxy to prevent them from taking it over. They were insectoid scavengers, whose physiology resembled that of army ants and the armor of the samurai.
According to the manual for Star Trek: Klingon Academy, the Klingons acquired warp drive from the Hur'q, by capturing and reverse-engineering some of their ships when they resisted their incursions. This explained how the Klingons, despite not being focused on technological research, managed to make the societal jump to interstellar travel.