Korath in 2404.
Korath was a Klingon scientist. In an alternate timeline's 2404, Korath had successfully developed a prototype chrono deflector. Admiral Kathryn Janeway sought the deflector for her mission to alter the timeline, and return USS Voyager home in 2378, and applied pressure to get Korath a seat on the Klingon High Council in exchange for the deflector. However, Korath attempted to alter the deal, and gain access to the Federation shield generator technology. As a result, Janeway stole the deflector, making Korath pledge vengeance against her for the theft. (VOY: "Endgame")
Background
Korath was played by Star Trek veteran Vaughn Armstrong. Korath also appeared in the Star Trek: The Experience attraction, again played by Armstrong.
Apocrypha
In the storyline from Star Trek: The Experience, Korath held a deep hatred of Captain Jean-Luc Picard. Using a cloaked temporal distortion, Koroth broughta group of early 21st century humans into the 24th century. One of these humans was Picard's ancestor. Korath's plan, however, was disrupted by the USS Enterprise-D, and the 21st century humans were returned to their time.