- "When your power eclipses mine I will become expendable. This is the Rule of Two: one Master and one apprentice. When you are ready to claim the mantle of Dark Lord as your own, you must do so by eliminating me."
- ―Darth Bane to Darth Zannah[src]
The Rule of Two doctrine was the guiding principle of the Order of the Sith Lords after the internecine New Sith Wars. It replaced the practices of Lord Kaan's Brotherhood of Darkness, which claimed equality among Sith Lords and forbade the use of the title "Darth".
The Rule of Two was instituted by Darth Bane, a former student at Kaan's Sith Academy, in response to what he viewed as inevitable, self-destructive infighting within the Sith Order. The Rule of 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.
- "Two there should be; no more, no less. One to embody power, the other to crave it."
- ―Darth Bane[src]
Darth Revan trained only one personal apprentice, Darth Malak.To avoid the seemingly inevitable clash between power hungry Sith, Darth Bane sought to reinvent the Sith Order with only two members—a Sith Master, and an Apprentice.
Bane formulated the concept of the Rule of Two after discovering a holocron in the Temple of the Ancients on Lehon. The holocron belonged to the ancient Sith Lord Darth Revan, who had refused to train more than one apprentice, believing those who did so were fools. Indeed, though Revan's Sith Empire included a Sith Academy and the training of multiple Warriors, Acolytes, and other practitioners, there were only two true Sith Lords, Revan and Malak. This is similar to the older Sith Empire of Exar Kun and Ulic Qel-Droma, though the two were equals instead of Master and Apprentice and Kun trained multiple apprentices simultaneously. With this lesson from Darth Revan's Holocron in mind, Bane surmised that he could avoid further intra-Sith conflict by applying Revan's philosophy to the Sith Order as a whole, limiting the Order to only one pairing of Master and Apprentice.
His course of action decided, Bane destroyed the old Sith Order, the Brotherhood of Darkness, by tricking Lord Kaan into using a Force technique known as the Thought bomb, killing every Sith Lord in the Order but Bane. Bane then hid from the Jedi and invoked the Rule of Two. Through these actions, Bane fulfilled the prophecy of the Sith'ari and ended the New Sith Wars.
For nearly 1,000 years, the Sith watched and waited, doing their best to destabilize the Galactic Republic without revealing themselves. Around 188 BBY, during the Dark Jedi Conflict, Kibh Jeen told the Jedi that the Sith had survived and were operating under the Rule of Two, though his warnings were ignored or passed off as the rantings of a madman. Finally, one Sith Lord, Darth Sidious, under the guise of Senator Palpatine, was elected Supreme Chancellor and masterminded a devastating galactic war involving the Republic, and, as Emperor of the newly formed Galactic Empire, brought the Galaxy under the control of the Sith, bringing about their foretold revenge. Darth Sidious and Darth Maul, 32 BBY.:"Always two, there are. No more, no less. A master, and an apprentice."
It is worth noting that Sidious broke the Rule of Two at least twice: once by training Darth Maul while still under the tutelage of Darth Plagueis; and secondly, by taking on two apprentices at once, and in the form of two Jedi: Count Dooku, dubbed Darth Tyranus, and the Fosh Jedi Vergere. However, Vergere was said to be merely a candidate and not a "true" apprentice. It should also be noted that this information derives from Lumiya's account to Jacen Solo, and is the only known source. In the latter case, Vergere left Palpatine's instruction, while in the former, Sidious corrected his violation true to Sith form: by murdering his Master. It also appears that Sidious approached Tyranus prior to Maul's death, although details on the timing are vague. However, these could be considered careful precautions to ensure the continuation of the Sith, even if they did, technically, violate the Rule in its strictest form.
Sidious, Tyranus, and Vader trained and employed a number of Dark Jedi and other Force-sensitive agents such as the Dark Acolytes, the Inquisitorius and the Dark Side Elite. As these agents were not taught the secrets of the Sith and were considered merely acolytes, not apprentices, they did not consider this a violation of the Rule of Two. However, the fact that Tyranus sought to replace the Jedi Order with a Sith army, the so-called Fist of the Empire, indicated that he had envisioned the abolition of the Rule of Two once the Sith took over the galaxy. Much like Sidious had done by with Darth Maul, Vader also took on a secret apprentice of his own. Darth Sidious and Darth Vader, master and apprentice.Darth Bane, however, overlooked one possibility: that one of the Sith Lords may be redeemed. When Darth Vader killed Sidious, it appeared that there were no others to carry on the Order's traditions and teachings, and the Rule of Two was veritably destroyed. Once again, the Sith had destroyed themselves—or so it seemed. Another secret apprentice of Darth Vader, Lumiya declared herself Dark Lady of the Sith and took a succession of apprentices—at least Flint, Carnor Jax
However, after the Galactic Civil War, the New Republic and New Jedi Order were opposed by the Disciples of Ragnos, which nearly resurrected the spirit of Marka Ragnos on Korriban in the Valley of the Dark Lords. This institution broke the Rule of Two, as it had been broken by the Prophets of the Dark Side.
The Prophets were destroyed when the Dark Lady Lumiya and her apprentice Carnor Jax enlisted the aid of the dark warrior Azrakel who arrived at the headquarters of the prophets with vengeful fury, slaying a number of them, including Kadann himself before he fell. Lumiya and Jax arrived moments later and slew any remaining prophets.