Klingon mythic-historic figure, a great warrior who united the Klingon Empire 1,500 years ago.
The story of Kahless is a cornerstone of Klingon mythology and religion. (Birthright, Part II [TNG])
Legend has it that the messianic Kahless fought the tyrant Molor, whom he killed with the first bat'telh or sword of honor.
Another of the epic tales of Kahless relates how he fought his brother, Morath, for twelve days and twelve nights because Morath had lied and brought shame to his family. (New Ground [TNG])
Klingons believe that warriors who die honorably join Kahless, who awaits them in Sto-Vo-Kor, the afterlife.
Just before his death, Kahless pointed to a star and promised that he would one day return there. Klingon clerics established a monastery on Boreth, a planet orbiting that star, where they waited for centuries for Kahless to return.
In the 24th century, the clerics of Boreth devised an elaborate scheme whereby preserved cellular material from Kahless was cloned to produce a replica of the original Kahless. The replica was programmed with all the ancient teachings and parables, and actually believed he was the real Kahless.
The deception was quickly discovered, but Worf, son of Mogh, pointed out that the new Kahless could be considered the rightful heir to the throne. With the support of High Council leader Gowron, the new Kahless was installed in 2369 as ceremonial Emperor of the Klingon people. (Rightful Heir [TNG])
Another, presumably less accurate, copy of Kahless was created by the Excalbians in 2269, when they were attempting to study the human concepts of "good" and "evil". (The Savage Curtain [TOS])
Worf had a statue in his quarters depicting the heroic struggle. (Reunion [TNG])
NOTES: Several episodes, notably "Birthright, Part II [TNG] and "Rightful Heir" [TNG], have revealed fragments of the legend of Kahless, but the entire story still remains to be told.
Kahless as seen in "The Savage Curtain" [TOS], when he was played by Robert Herron, appeared very differently from Kahless as seen in "Rightful Heir" [TNG] when he was played by Kevin Conway. One might rationalize that this might have been because the image of Kahless created by Yarnek was drawn from the mind of James Kirk, who may not have known what the "real" Kahless looked like. Of course, in truth "The Savage Curtain" was filmed many years before "Rightful Heir," before the introduction of the more elaborate makeup design for the Klingon in Star Trek: The Motion Picture.