The Sword of Kahless was the first bat'leth ever forged. It was designed and created by Kahless the Unforgettable and was dated to the 9th century AD. According to Klingon mythology, Kahless created the sword by dropping a lock of his hair into a river of lava from Kri'stak's summit, 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 later used 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' 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")
The Sword of Kahless was based on the Holy Grail. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 110) Richard Danus was provided the concept of the Sword of Kahless as the genesis of the installment bearing the same name. "They had an idea for the sword of Kahless [to be featured in the story]," he remembered. "From that premise it was relatively easy to reach a conclusion that the missing sword will be a quest, a search for the sword." (Cinefantastique, Vol. 28, No. 4/5, p. 39) Hans Beimler, who wrote the episode's teleplay, stated, "The 'McGuffin,' if you will, stayed the same: we were always pursuing the Maltese Falcon – or, in this case, the Sword of Kahless. The big decision that was made in the story break session was not to make it so much of a quest, but to get to the bat'telh and see the effect it has on them." (The Official Star Trek: Deep Space Nine Magazine issue 15, p. 50) The fictional item became vital to the installment, Ira Steven Behr commenting, "If the audience isn't made to understand the spiritual importance of that weapon, then we have no episode." (Star Trek: Deep Space Nine Companion (p. 291))
When the Sword of Kahless is introduced in the episode's script, the teleplay describes it as "old, rusty, primitive" but "magnificent." 
The writers considered the possibility that the sword might hold some sort of mystical or chemical power over those who touched it. However, they wanted the sword's effect to be internal, not external, and didn't want to draw responsibility away from the characters, such as by making an external influence the real culprit. (Cinefantastique, Vol. 28, No. 4/5, pp. 39-40) Hence, Hans Beimler chose not to imbue the sword with miraculous powers, despite the Holy Grail having generated centuries of tales of virtually supernatural power. "The idea is that the sword itself doesn't have any magic," he clarified. "It's the concept of the sword that has the power [....] The minute anyone starts talking about the sword it starts infecting them, so Worf gets caught up from the very beginning [of the narrative]." (Star Trek: Deep Space Nine Companion (p. 290)) The writers also knew they wanted the sword to eventually be discarded by the characters originally seeking it. "We knew in order to sell that, we had to show that this was indeed the Holy Grail for these guys," said René Echevarria. "If their religion in the largest sense is one of battle, victory, personal glory and power and you've got this in your hands, then it starts to eat away at you and you start to talk about your destiny." (Cinefantastique, Vol. 28, No. 4/5, p. 40)
Worf actor Michael Dorn proposed finding, after the Sword of Kahless is beamed into space at the end of the episode sharing its name, that the real Sword of Kahless was secretly being kept by Worf and that he had taken it back to his quarters. René Echevarria was critical of Dorn's suggestion, noting, "This Holy Grail is just too dangerous for the Klingons now; put it to destiny and maybe someday their people will be ready to do it honor." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 110)
Because Klingon bat'leths had been established for numerous years, Kahless' sword obviously needed to look special. Countless concept drawings of the artifact were illustrated, one such image sketched by John Eaves and dated September 1995. The sword's design was refined by Visual Effects Producer Dan Curry, who took only some of the elements from Eaves' renderings and added a triangular midpoint to the weapon. (Star Trek: Deep Space Nine Companion (pp. 291 & 289)) Curry remarked, "The Sword of Kahless was like a baroque version of a bat'leth, where it had different surfaces [so] that it would be easier to grapple, and the central disembowelment fringe." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
The concept artwork was supplied by the DS9 art department to Joe Longo. Starting with the illustrations, he ordered a new bat'leth constructed of hardened aluminum, featuring a leather-wrapped handle. Next, the prop was given to freelance sculptor Dragon Dronet. He etched designs into the blade using a handheld dental bit. The producers wanted the aluminum to look old and quite similar to Damascus steel; Dronet chose to make the patterns seem like a mix of Damascus steel and a topographical map, wanting to make it look as if the blade showed mountains as viewed from above. He then wrote Klingon names down the side of the blade. (Star Trek: Deep Space Nine Companion (pp. 291-292))
Dragon Dronet took part in the creation of not only the Sword of Kahless prop itself, but also a stand for the item. In fact, he designed the stand from scratch and built it himself. "I made the stand out of one-inch-thick Plexiglas," explained Dronet, "and engraved the pattern into it." He lastly spray-painted the stand, making it appear metallic, and carved its legs to make them look like the feet of targs, a finishing touch he thought was funny. (Star Trek: Deep Space Nine Companion (p. 292))
Reception and trivia
Ira Steven Behr approved of how the Sword of Kahless is depicted in the episode of the same name. "I did [...] think the whole idea of the sword – the whole Treasure of Sierra Madre thing – was really nice," he reminisced. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 110)
There was a great deal of online chat about the Sword of Kahless. "I noticed a lot of discussion on the Net about it," stated Hans Beimler, "and it was interesting that people assumed that there was a virus or something that infected them when they touched the sword." (Cinefantastique, Vol. 28, No. 4/5, p. 40) The writing staff was disappointed to discover that some viewers couldn't accept the notion that the sword had no external powers. René Echevarria expressed his opinion of the Internet conversations; "A lot of fan reaction was that there must be a tech explanation, that the sword was omitting something. I was astonished." (Star Trek: Deep Space Nine Companion (p. 290))
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.
In the PC game Star Trek Online, Emperor Kahless gives KDF players a replica of the Sword of Kahless after recovering the original. The Emperor carried the original sword in battle against the Iconians, severing the arm of the Iconian leader T'Ket with it before she kills him in single combat. In the expansion "Victory is Life", Grand Nagus Rom dispatches a team of Ferengi, led by his brother Quark, to reclaim the Sword from T'Ket's flagship and return it to the Klingons, in order to convince the Empire to fully commit to the conflict against the Hur'q. Examination at DS9 shows that the Sword is made of from a 4,000-year-old Hur'q carapace, which is naturally infused with germanium. The Sword is then delivered (after being "stolen" by Leck) to Grilka, who brings it to Martok. After Worf becomes the Torchbearer and lights the Beacon of Kahless, Martok leads the united Klingon fleet to the Founders' homeworld of Empersa, and wields the Sword on the surface against the Hur'q seeking to destroy the Great Link. Martok continues to wield the Sword through another Klingon civil war against the insane self-declared Emperor J'mpok until he is wounded by Aakar, the grandfather of Gowron; he then surrenders the blade willingly to the clone of L'Rell, who later becomes the new Chancellor.