Kahless, or Kahless the Unforgettable, is the name taken by two political leaders in the history of Klingon society.
The original Kahless was a Klingon warrior who lived sometime during the 9th century AD. Following a series of battles, he united the Klingon people and founded the Klingon Empire with himself as the first Emperor.
After his death, Kahless acquired a near-mythical status and was revered much like a religious figure. His descendants maintained positions among the High Council, and his achievements were immortalized in a canon of folk tales and songs.
Kahless was said to have predicted his own return at some point in the future to once again lead the Klingon Empire.
A figure claiming to be Kahless appeared in a Klingon monastery in 2369. While DNA tests proved he was at least related to the original Kahless, he had very little memory of his past life beyond what was widely known through myths. Eventually this Kahless was exposed as a fraud, a clone created by Klingon priests in an attempt to give the Klingon people a figurehead they could rally behind; this revelation did not stop the Kahless clone from assuming the title of Klingon Emperor, albeit in a purely ceremonial fashion.