The legendary first emperor of the Klingon Empire, Kahless not only united his people some 1,500 years ago, but he also set the standard for the conduct of honor and strength all Klingons live by. All Klingon children learn the tales of Kahless and his brother, Morath, who fought for 12 days and 12 nights because Morath had broken his word and dishonored his family, killing their father. According to legend, Morath then threw their father's sword into the sea to prevent Kahless from having it, but he held his breath for three days and found it. As an adult, Kahless forged the first bat'leth and founded the Empire after slaying the tyrant Molor — a battle celebrated with the Kot'baval festival — and conquering the Fek'Ihri.

Legend also tells of many other exploits: how he once fought off an entire army by himself at Three Turn Bridge, skinned the serpent of Xol, and carved a statue for his beloved Lukara. As his first wife, Lukara and Kahless met after fending off 500 warriors at the Great Hall at Qam-Chee, when the city garrison fled. Their vows in that incident are one of the most romantic Klingon rituals of mating ever, often recreated by modern couples. After delivering the laws of honor in later life, Kahless went to the edge of his city and made 'The Promise' to his weeping people that he was leaving to guard the afterlife of the honored dead, in Sto-Vo-Kor, but would return someday.

That night, on the planet, Kahless pointed out Boreth, and a caste of clerics sprang up to await the day he would return. Only they knew how he created the first bat'leth from a lock of hair, or to use the tale of Mount Kristak to verify his identity upon return. In 2369, when a clone of Kahless appeared, taken from blood on the Knife of Kirom, was created and allowed to serve as perfunctory emperor and spiritual.