There are a few universal truths in the Star Wars universe. Han Solo never wants to know the odds, cantinas will always be full of colorful characters, and a Sith will always fall at the hands of his apprentice. This was true of Vader and Palpatine, of Palpatine and his master, Darth Plagueis, of Plagueis and his master, Darth Tenebrous, and so on throughout the ages. It wasn’t always this way, though. One Sith established this rule, and in doing so, shaped the Star Wars universe as we know it.
Darth Bane has always been waiting in the wings of Star Wars canon. He was born a millennium before the events of the original trilogy, but was an important character nonetheless. In the non-canon Star Wars works, its was established he was a young Sith upstart who got disillusioned with the Sith leadership and forged his own path, similar to the heel-turn Anakin would take with the Jedi much later.
Bane overthrew the Sith Brotherhood, the Sith governing body, and established the Sith Order that would remain everafter. Fed up with all the ambitious infighting, he established the Rule of Two: there can only be two Sith at any one point, and the master will always die at the hand of the apprentice.