The Sword of Kahless was the original bat'leth forged by Kahless the Unforgettable, a priceless relic lost for a thousand years after the Hur'q raided the Klingon homeworld.
Klingons long believed that whoever possessed the sword could unite the Empire. It was taken when the Hur'q invaded Qo'noS centuries ago and was never recovered, becoming a legendary goal of quests.
In 2372 Kor, Worf and Jadzia Dax tracked the sword to a planet in the Gamma Quadrant and found it. The power of the relic soon drove Kor and Worf to quarrel over which of them should present it, and they realized it would divide rather than unite the Empire. They beamed it into space, where it might be found when the time was right.