Molor was the "leader" of the Klingon people from about 450 to 500. He was a tyrannical dictator during that time, and many rebel groups sprung up to try to drive him away. However, the rebels rarely worked together, and thus, Molor was able to stay in power for quite some time. Kahless, who proved to be Molor's downfall, was able to bring several rebel groups together to change the balance of power.
As seen in the Star Trek: The Next Generation episode Firstborn, Kahless himself ultimately kills Molor with The Sword of Kahless, the first Bat'leth invented. Their fight is popular among Klingon re-enactors of the 24th century.
Kahless then founded the Klingon Empire, which turned into a thriving, star-faring race.
More information is contained in the novel Kahless by Michael Jan Friedman.