AT: "xx" Korath was a Klingon scientist who, in an alternate 2404, created the chrono deflector, a time travel device.
In that year, Admiral Kathryn Janeway entered into negotiations with Korath to obtain the chrono deflector, as part of her plan to change history by returning the USS Voyager to the Alpha Quadrant sixteen years earlier than it had in that timeline. Korath agreed to provide Janeway with the deflector in exchange for the admiral using her influence to secure Korath a seat on the Klingon High Council. Janeway honored her end of the agreement, but Korath refused to give her the deflector unless Janeway also provided him with ablative generator technology. In response, Janeway stole the deflector and evaded Korath's ships in SC-4, her Starfleet Command shuttlecraft. Using the deflector to generate a temporal rift to the year 2378, Janeway was successful in altering the timeline and returning Voyager to Earth in that year. (VOY: "Endgame")
Appendices
Background information
Korath was played by Star Trek veteran Vaughn Armstrong. Shortly after portraying recurring character Admiral Forrest for the first time in ENT: "Broken Bow", Armstrong said of the earlier role, "I like to think of Korath [...] as a mad scientist who loved to play with these toys of destruction. He was a warrior who had lived beyond that age and had become a kind of consultant and turned all his efforts into making better weapons, and he'd gone mad doing that. But at a convention some Klingons told me they thought he was a bit like a Ferengi because he was without honor in his dealings with Janeway; he changed the deal midway. But the way I look at it is that we hadn't quite made the final deal. And an episode I would love to see would show Korath taking the time machine that Janeway stole from him and going back to stop Admiral Forrest from sending people out into space!" (Star Trek: The Magazine Volume 2, Issue 9, p. 44)
Apocrypha
Korath first appeared in the 1998 Star Trek: The Experience attraction, the Klingon Encounter, set in the year 2371, in which he was also played by Vaughn Armstrong. He described this form of the character as "young and all-out," also revealing, "I asked them in the audition, 'We're not talking about anything subtle here, are we?' And they said, 'No, go for it!' and I did!" (Star Trek: The Magazine Volume 2, Issue 9, p. 44)
According to LCARS data that flashed across one of the workstations on the bridge of the Enterprise in the Klingon Encounter, Korath is from the House of Monak and is the captain of the ISS Boreth, a B'rel-class Bird-of-Prey. He achieved command following the assassination of K'holt and was recognized for his part in the Battle of Khitomer, earning the Sword of Honor and the Kahless Medal. Korath developed a deep hatred of Captain Jean-Luc Picard back in his days as a cadet in the Klingon Imperial Corps after coming in a close second to then-ensign Picard while competing in the Federation "Outreach Games." News reports indicated that Picard attempted to congratulate Korath after the awards ceremony, only to have Korath spit in his hand and swear vengeance for "the dishonor before the entire galaxy."
While none of this is explained during the attraction, Korath's plot for revenge is the focus of the story. Using a cloaked temporal distortion, Korath attempted to abduct a group of early 21st century Humans into the 24th century. One of these Humans was Picard's ancestor, and Korath had planned to erase Picard from the timeline. Korath's plan, however, was disrupted when the USS Enterprise-D, acting on information from Federation Intelligence, intercepted his transporter beam, and returned the 21st century Humans to their own time in a shuttlecraft. Korath's ship was destroyed by the Enterprise in the skies over Las Vegas after both vessels followed the shuttlecraft back in time.
Korath later appeared in the novel Watching the Clock, in which he is depicted as being the foremost temporal physicist in the Klingon Empire. In the novel, it is implied that Korath was the man whom the adult Alexander Rozhenko met in the Cambra system in an alternate timeline in the early 25th century and who offered him the chance to travel back in time to 2370, as was depicted in TNG: "Firstborn".