The Sword of Kahless was the first bat'leth, forged by Kahless the Unforgettable around the 1st millennium AD. According to Klingon mythology, Kahless created the sword by dropping a lock of his hair into lava from the Kri'stak volcano, and then cooling the burning lock in the Lake of Lusor and forging it into a blade. (TNG: "Rightful Heir")
Brandishing his newly-forged Sword, Kahless slew the hated tyrant Molor, conquered the Fek'Ihri, and created the first Empire. Kahless would later use the Sword to skin the serpent of Xol, to harvest his father's field, and to carve a statue of his beloved, the Lady Lukara.
After Kahless's death, the Sword became a revered object among the Klingon people. The Sword was wrapped in a special cloth, the Shroud of the Sword, that was also a revered object. However, both the Sword and the Shroud were stolen about five hundred years later by the Hur'q during their invasion and plundering of Qo'noS.
Following the theft of the Sword, numerous fake Shrouds were "discovered" over the years. A Vulcan expedition found the true Shroud among the remains of a Hur'q outpost in the Gamma Quadrant in 2372. Using the data provided by the Vulcans, Dahar master Kor launched a quest to find the Sword itself. He, along with Jadzia Dax and Worf discovered the Sword in the same Hur'q outpost.
However, the enormous symbolism of the Sword became a matter of contention between Worf and Kor, as well as the target of theft by Toral, son of Duras. After realizing that the Sword would only serve to divide the Klingon Empire further rather than reunite it (as the legends foretold), Dax, Worf, and Kor set it adrift in interstellar space. (DS9: "The Sword of Kahless")
In the PC game Star Trek: Armada, Worf retrieved the sword from interstellar space in order to unite the empire against the forces of Toral.