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Darth Bane was active in the last era where the Sith were numerous. He saw how Sith plans were undone by infighting and scheming amongst the different Sith Lords and upon the destruction of the Sith Order, created a new doctrine: The Rule of Two. Bane's idea was that there could only be two Sith at any one time: one Master to hold power and knowledge, and one Apprentice to crave the Master's knowledge and bring it to the next generation of Sith in secret, slowly undoing the works of their ancient enemies.
Following his doctrine, Bane took on only one apprentice. Having trained this individual to follow his doctrine and plot the downfall of the Galactic Republic and the Jedi Order from the shadows, Bane and his teachings were discovered by the Jedi and he was hunted down. Upon his death, the Jedi believed the Sith Order to be destroyed for good. To commemorate the end of the Sith, the Jedi buried Bane in a sarcophagus on Moraband, the ancient homeworld of the Sith.
About 1000 years later, Jedi Master Yoda traveled to Moraband as part of his trials to learn the secret of eternal consciousness. There, he encountered an illusory specter of Darth Bane. The specter seemed to form from Bane's remains, reanimated by black Dark Side smoke coming from inside his sarcophagus. Bane's smoldering specter tempted Yoda with becoming his apprentice, but the Jedi Master resisted his suggestion by declaring Bane to simply be an illusion and having nothing to teach him. Deprived of the power of fear and intimidation, the specter was pulled into a hole on the side of the sarcophagus and disappeared from view.
Darth Bane's legacy was eventually fulfilled by Darth Sidious, a Sith Lord who took control of the Galactic Republic as an unassuming politician and brought the galaxy into the conflict known as the Clone Wars. Sidious transformed the Republic into the Galactic Empire, simultaneously framing the Jedi as traitors to the state and having them hunted to near-extinction.