(covers information from several alternate timelines)
Resembling a crescent-shaped, two-ended scimitar, the bat'leth was wielded using three handholds along the outside edge of the blade. (TNG: "Reunion", et al.) A traditional bat'leth was typically approximately 116 centimeters long, with blades of composite baakonite, weighed about five-point-three kilograms, and had an exterior hand grip diameter of five centimeters. (DS9: "Blood Oath")
According to Klingon mythology, the first bat'leth was forged by Kahless the Unforgettable in the 9th century. Kahless cut a lock of his hair and dropped it into the lava of the Kri'stak volcano, then plunged the burning lock into the Lake of Lusor and twisted it into a blade. After forging the weapon, he used it to fight the tyrant Molor, and then gave it its name. This story was not recorded in public texts, but was passed down verbally by the Klingon clerics as a test of Kahless' return. (TNG: "Rightful Heir") (See also: The Story of the Promise)
The Sword of Kahless was preserved by the Klingons following Kahless' death, until it was stolen by the Hur'q when they plundered Qo'noS about five hundred years after his death. (DS9: "The Sword of Kahless")
In the alternate reality, at least two Klingon warriors wielded bat'leths in a battle between a Klingon patrol, a USS Enterprise away party led by James T. Kirk, and "John Harrison" in the Ketha Province. (Star Trek Into Darkness)
- "Blood Oath"
- "The House of Quark"
- "The Way of the Warrior"
- "The Sword of Kahless"
- "Sons of Mogh"
- "Bar Association"
- "Looking for par'Mach in All the Wrong Places"
- "Nor the Battle to the Strong"
- "Sons and Daughters"
- "You Are Cordially Invited"
- "Image in the Sand"
- "Tacking Into the Wind"
Although this weapon has become known as a bat'leth, it was initially to be named a bat'telh, which the pronunciation guide from the final-draft script for TNG: "Reunion" recommends be pronounced "BAT-telth". The scripts which refer to it with this name include not only that for "Reunion" but also those of "Night Terrors", "New Ground" and "The Quality of Life".    
In non-canon Klingonese devised by Marc Okrand, the Klingon word for this weapon is betleH, etymologically derived from batlh 'etlh (literally translated as "glory blade"). Stated "Reunion" co-writer Ronald D. Moore, "I think I pulled 'bat'leth' from the dictionary by combining, like, the word for 'sword' and the word for 'honor,' and I think I literally combined them into 'bat'leth.' So, I think that was kinda rooted in Marc's language." ("Reunion" audio commentary, TNG Season 4 Blu-ray)
Introduced in "Reunion", the bat'leth was originally inspired by comments from Worf actor Michael Dorn, as he wanted his character's fighting style to be more martial arts than barbarian, which synced well with writer Ronald D. Moore thinking of the Klingons as akin to samurai. (Star Trek: Communicator issue 114, p. 59) "You know, there was this desire to have a particular, cool Klingon sword for Worf to use, that would become his, sort of, signature weapon," remembered Moore, "and Michael Dorn had actually always asked for this. He would always, like, say that he wanted a cool weapon that was just Worf's." ("Reunion" audio commentary, TNG Season 4 Blu-ray) Dorn's remarks led Moore to think up the bat'leth, which he later described as "a more distinct, stylized weapon." After conceiving of such a prop, Moore consulted Dan Curry. (Star Trek: Communicator issue 114, p. 59) A martial arts expert, Curry not only designed the bat'leth but also helped develop the intricate moves performed in its use. He noted, "The script for 'Reunion' called for a special Klingon bladed weapon." (Star Trek: The Next Generation 365, p. 178)
Dan Curry wanted the new Klingon weapon to look relatively unique. "I looked at some of the designs that were proposed and they were wonderful, but there was a degree of familiarity about them. And I thought to myself, 'Well, you know, it's not really my job to get in people's faces about this, but because I care passionately about martial arts, and I have a strong feeling for the ergonomics of bladed weapons, let's have something that we've never seen before." (Star Trek: The Next Generation - The Continuing Mission, p. 179)
Having always been irritated that films seemed to keep portraying weapons that were meant to look appealing but actually couldn't be handled practically, Dan Curry had been envisioning a pragmatic style of weapon for a long time, thinking of it as basically a staff weapon infused with Oriental influence. (Star Trek: The Next Generation 365, p. 178; Star Trek: The Next Generation - The Continuing Mission, p. 179) Explained Curry, "I'd been imagining a curved weapon partially influenced by Himalayan weapons like the kukri [the wickedly curved knife of the Gurkhas of Nepal, arguably the most renowned fighting knife in the world]. I was also thinking about the Chinese double ax, Chinese fighting crescents, and the Tai Chi sword. I combined elements of all those things in order to come up with an ergonomically sound weapon." (Star Trek: The Next Generation 365, p. 178) It was also important to Curry that the bat'leth could be held with either two hands or one hand. "He wanted a weapon that you could hold, you know, in a variety of ways," offered Ron Moore, "and have some interesting angles, on camera." ("Reunion" audio commentary, TNG Season 4 Blu-ray)
Dan Curry then began to put his ideas for the weapon into practice, sketching it out on a Postit note. (Star Trek: Communicator issue 114, p. 59) A small pencil drawing matching the eventual shape of the weapon but with very minor differences, this illustration was "on the back of a Paramount phone message note," in Ron Moore's words, and was sketched in Moore's office. ("Reunion" audio commentary, TNG Season 4 Blu-ray) As such, the note ended up in his possession. (Star Trek: Communicator issue 114, p. 59; "Reunion" audio commentary, TNG Season 4 Blu-ray) Once at his own home, Curry crafted a foam-core mock-up of how he envisioned the weapon. He proceeded by showing this item to Executive Producer Rick Berman. (Star Trek: The Next Generation - The Continuing Mission, p. 179) The foam-core version of Curry's design was an admittedly flimsy prototype but was granted approval. "And Rick liked it," Curry recalled. (Star Trek: The Next Generation 365, p. 178)
The final draft of the "Reunion" script describes the weapon by saying, "It is [a] semi-circular curved blade that branches to four points. It is about three feet wide. There is a terrible beauty about the blade [....] The bat'telh is held by two handles on the mid-exterior of the blade [....] The bat'telh can be used as either shield or sword." 
During at least one production meeting, Dan Curry became really animated with demonstrating how the bat'leth could be held, swung and used to disembowel someone. Michael Okuda, who was another of the meeting's participants, noted with a laugh, "We were all a little taken aback." ("Reunion" audio commentary, TNG Season 4 Blu-ray)
At the time of its creation, nobody knew that the bat'leth was to later become a type of symbol for the Klingon species. (Star Trek: The Next Generation 365, p. 178) Over the years between its introduction in "Reunion" and its appearance in Star Trek Into Darkness, however, the bat'leth has become very well established. "There's a lot of history about the Bat'leth," commented Andrew Siegel. "It's a prop that Trekkies really like." (Star Trek Magazine issue 173, p. 79) With a smile, Dan Curry observed, "Now you seldom see a picture of a Klingon without a bat'leth in his hands." (Star Trek: The Next Generation 365, p. 178)
The bat'leth appeared in the background in the Stargate SG-1 sixth season episode "The Other Guys". John Billingsley, who played Dr. Phlox on Enterprise, also appears in the episode, as a scientist obsessed with Star Trek.