Rule of Two

The Rule of two is a principle in the fictional Star Wars universe used to describe the Sith practice of allowing only two Sith Lords to exist at a time: a Master and an apprentice. Observed Yoda,

Always two there are; no more, no less: a master and an apprentice.

For the Sith, like the Jedi, training is based on a unit of two people: one Master and one apprentice. No more, no less. The Master trains the apprentice in everything that he knows of the dark side of the Force. Unlike the Jedi, though, who peacefully graduate from Padawan to Jedi Knight and then Jedi Master based on merit, the current Sith apprentice only becomes the Master once his own Master has died, either by accident, natural causes, or his apprentice's own hand. At this point, the new Sith Master seeks out his own apprentice, and the cycle begins again.

It is somewhat curious as to why a Sith Master would take an apprentice, knowing that, eventually, that apprentice will probably kill him. Perhaps the pride of knowing that the apprentice, the one the Master himself trained, managed to succeed him in mastery of the Force is reward enough. This is supported by Star Wars Episode III: Revenge of the Sith, where Emperor Palpatine is quoted as saying to Yoda: "Darth Vader will become more powerful than either of us!"

The Rule of Two was first instituted by Darth Bane approximately 1,000 years BBY, following the cataclysmic Battle of Ruusan. Prior to the destruction of the latest incarnation of the Sith Order in that battle, the possible number of Sith was unrestricted, as it had been for over 24,000 years, since the days of the ancient Sith Empire. Darth Bane, Ruusan's sole surviving Sith Lord, realized that the Order would have to be reformed if it was to survive and seek its revenge on the Jedi. After choosing an apprentice of his own, Darth Zannah, Bane fled to the tomb of ancient Dark Lord of the Sith Freedon Nadd on the Onderonian moon of Dxun. In the tomb, Bane studied an ancient Sith Holocron extensively, until he underwent a transformation that would determine the future of the Sith Order.

In reforming the Sith, Darth Bane looked back on the Order's past incarnations, analyzing the strengths and weaknesses of each and determining where they went wrong. The most important thing he realized was that, the greater the Sith's numbers, the more self-destructive they became. The ancient Lords of the Sith Empire had warred for centuries, and the Brotherhood of the Sith, prior to Lord Kaan's unification of the Order, had been similarly divided. At the final Battle of Ruusan, the Brotherhood had been utterly destroyed not by the Jedi, but by Kaan himself, via a suicidal thought bomb. Perhaps taking inspiration from the legendary Exar Kun/Ulic Qel-Droma and Darth Revan/Darth Malak Sith duos, Bane decided that the dark side of the Force should be concentrated in only two Sith Lords: a Master and an apprentice. And so the thousand-year tradition was born.

The Jedi Order apparently did not know of the continued existence of the Sith until the appearance of Darth Maul on Tatooine shortly before the Battle of Naboo. Nonetheless, Master Yoda was already aware of the Rule of Two at that time. Darth Maul was the apprentice of Darth Sidious, who had himself been the apprentice of Darth Plagueis (and apparently killed him). After Maul's death, Sidious took the fallen Jedi Count Dooku as his new apprentice, giving him the name Darth Tyranus.

The continued danger to the life of one Sith from the other was demonstrated when Chancellor Palpatine, who was secretly one and the same person as Sidious, encouraged Anakin Skywalker to kill Dooku. Sidious had apparently never regarded Tyranus as his true successor; his long-term plan had been to use Dooku as the victim of Anakin's "first cold-blooded murder" (the Star Wars Episode III: Revenge of the Sith novelization by Matthew Stover) and convince Anakin to join the Sith as his apprentice and heir apparent. When Anakin accepted the name Darth Vader, the most legendary example of the Rule of Two appeared to terrorize the galaxy: Emperor Palpatine/Darth Sidious, the Sith Master, and Darth Vader, the Sith Apprentice.

This partnership was itself not without internal tension. Within days of becoming a Sith, Vader suggested to his wife Padme Amidala the possibility of his killing the Emperor and taking over from him. Years later, Vader proposed to his son, Luke Skywalker, that they kill the Emperor together; Luke would then have become Vader's apprentice, preserving the Rule of Two. Palpatine also hoped for Luke to kill Vader and become his own apprentice.

With Palpatine and Vader's deaths at each other's hands over the moon of Endor, the Rule of Two was ended. Palpatine's later returns from the dead and other attempts to revive the Sith were not accompanied by a formal return to the Rule of Two, although Palpatine did continue his attempts to make Luke Skywalker his apprentice.