A thousand years before Obi-Wan Kenobi gave Luke Skywalker his father's lightsaber, the Sith numbers rivaled the Jedi Order in the Brotherhood of Darkness. While the war between the Jedi and the Sith raged on, a young soldier named Darth Bane realized that the Sith's infighting and selfishness would be the reasons they could never beat the Jedi. To make the Sith more powerful, he destroyed them. As the sole survivor, he took on an apprentice, Darth Zannah.
Bane realized that the Sith could only remain powerful if they schemed and pulled strings to manipulate the Republic and destroy the Jedi from the shadows. To do that, he created the rule of two: "There should only be two, no more, no less. One to embody power, the other one to crave it." If you are a Sith, the only way to attain power is to take out the strongest existing Sith, the very one who taught you everything you know. The result is the Sith will only ever be ruled by the strongest, and the practice enabled them to undermine the Republic and eventually create the Galactic Empire.
Precisely as he intended, Zannah was able to lure him to the planet of Ambria, where she killed him and became the last Sith. His death and the Jedi's ignorance of Zannah's existence ushered in a millennium of peace where the Sith stayed hidden until Darth Maul (Ray Park) dueled and murdered Qui-Gon Jinn (Liam Neeson).
