In the universe of the Star Trek television shows and films, Kahless the Unforgettable was the legendary founder of the Klingon Empire, the inventor of the sacred bat’leth sword and Klingon-style martial arts, and a messiah-like figure at the center of Klingon religious beliefs.
According to legend, Kahless brought unity to the Klingon people some 1,500 years ago by killing the evil tyrant Molor with the very first bat’leth - a sword-like weapon of his own invention which he forged by dipping a lock of his own hair into the lava of the Kri'stak Volcano and fashioning it into a blade.
According to myth, when Kahless had accomplished everything he needed to, he voluntarily departed this universe for the Klingon afterworld of Sto-Vo-Kor, but before his departure he promised his tearful followers that he would return one day, in the Klingon Empire's hour of need. Pointing to a star in the sky, he told his followers to look for him there, on that point of light.
The planet Boreth, which orbited that star, became a popular pilgrimage destination for followers of Kahless over the centuries. In the 24th century, the clerics of the monastery on Boreth contrived to clone Kahless using DNA on a knife that had once stabbed the real Kahless, and claim that Kahless had returned as promised (TNG: "Rightful Heir"). Although this subterfuge was ultimately discovered, the clone of Kahless went on to become the Emperor of the Klingon Empire.
Earlier, in the 23rd century, the USS Enterprise under Captain James T. Kirk had encountered an image of Kahless projected by Excalbians as part of their experiments into the nature of good and evil (TOS: "The Savage Curtain"). Unlike the real Kahless, the image of Kahless had the smooth forehead found only on Klingons who were infected with the Klingon Augment Virus, and a 23rd century Klingon standard-issue fleet uniform. However, this was obviously only because the image was drawn from the minds of the Enterprise crew, who had never seen an image of the real Kahless.