Darth Bane

Unless you are true Star Wars geek and read the novels you never heard of Darth Bane. Darth Bane was the Dark Lord Of Sith responsible for Rule Two. Rule Two states that there would be only two Sith at one time, a Master and an Apprentice, guaranteeing that when the Apprentice became capable of usurping the Master, they would be worthy of the title. The former would take the latter's life and role, and begin the cycle anew. Both Master and Apprentice were considered to be Sith Lords. Here is a little history taken from Wookieepedia.

History -
Darth Bane, born under the name of Dessel, was the Sith'ari and the Dark Lord of the Sith responsible for creating the Rule of Two. Born in 1,026 BBY, he was raised as a poor miner on the Outer Rim planet Apatros. After killing a Galactic Republic ensign in a fight in 1,003 BBY, he was in danger of being arrested by the Republic. With the help of his friend Groshik, Dessel escaped off-world to join the Sith Brotherhood of Darkness. Initially serving as a foot soldier in the Gloom Walkers, he was recognized as a Force-sensitive, and taken to the Sith Academy on Korriban.

He studied there for a time, quickly becoming one of the best students before he lost his faith in the dark side of the Force. After a short time, Bane, as he was now known, was able to regain his faith and determination to become the best. However, he would soon lose confidence in the Brotherhood of Darkness, believing its leader, Kaan, to be a coward and a fool. Deserting the order, he studied the holocron of Darth Revan, and, armed with new knowledge, helped destroy the Brotherhood, allowing him to create his own Sith Order. Bane then instituted a Rule of Two, which stated that there could be only two Sith to avoid the infighting that had plagued the Sith for millennia. He styled himself as Darth Bane, and took as his apprentice a Human female, whom he christened Darth Zannah.

In 990 BBY, he sought to learn how to create a holocron, through which he would pass down his knowledge to future Sith Lords. The Sith journeyed to the Deep Core world of Tython to locate the Sith holocron of the ancient Dark Lord Belia Darzu. However, while Bane was on Tython, the Jedi learned of his existence, and sent a group of Jedi to kill him and his apprentice, who had recently arrived to tell him that they were being hunted. Once the Jedi arrived, they confronted the two Sith in Darzu's fortress. Though outnumbered, the Sith were able to defeat the Jedi; however, Bane was grievously injured. Zannah took him to Ambria, where she convinced the healer Caleb to help them. Caleb notified the Jedi Council, only to have Zannah use her powers to drive Caleb's assistant Darovit insane. He then killed Caleb, and was killed by a group of Jedi when they arrived, as they believed him to be the Sith Lord. Thus, Bane's order was kept safe from the Jedi.

A decade later, Bane began to worry that his apprentice was too weak to overthrow him and assume the mantle of Dark Lord of the Sith, as was necessary under the Rule of Two. He began to research the secrets to prolonging his life by transferring his essence to another body, and traveled to Prakith, where he claimed the holocron of Darth Andeddu. However, while Zannah was returning from a mission on Doan, Bane was ambushed by a team hired by Caleb's daughter, Serra, and, grossly outnumbered, he was defeated and captured. Taken to Doan, Bane was imprisoned and interrogated, only to covertly gain his freedom a short time after. Bane was then attacked by Zannah, who sought to become the new Dark Lord, but he escaped and tracked Serra to Ambria with Darth Cognus, an Iktotchi assassin skilled in use of the dark side who Bane planned to take as his apprentice if Zannah proved herself weak and unworthy. After Cognus killed Serra, Bane, determined to test his apprentice for the last time, sent Zannah a message to meet him on Ambria. Following a heated duel, Bane attempted to transfer his essence into Zannah's body. However, her will proved to be too strong and Zannah defeated him, ending the life of the Sith'ari, yet also validating herself as his successor. Nearly a millennium later, Bane's Sith Order would defeat the Jedi and overthrow the Republic.

You can read about Bane in novels by Drew Karpyshyn.
Darth Bane: Path of Destruction,
Darth Bane: Rule of Two,
Darth Bane: Dynasty of Evil