Instead of two there is now only one—the Sith Order itself. I have recreated the Sith, Lord Bane, as you once did. I have given it a purpose. For what use is power without one?
The Rule of One was the guiding principle of the One Sith. Unlike previous Sith principles, the Rule of One focused on obedience
Power is its own purpose. To share it is to dilute it. You delude yourself, pretender -- Your order will yet turn on itself and you.—Darth Bane to Darth Krayt for disobeying the Rule of Two, Star Wars Legacy 5: Broken, Part 4
When Darth Krayt founded his new Sith Order, he abolished Darth Bane's Rule of Two, a dictate which commanded that there should be only two Sith at any one time; a master and an apprentice, one to embody the power of the dark side, the other to crave it. Instead, there would be the Rule of One—that One being the Sith Order itself, under which there would be many minions, acolytes/adepts and lords with only one Dark Lord of the Sith. Darth Krayt believed he gave the Sith and their power a purpose, unlike Darth Bane who believed that power was its own purpose.
Under the Rule of One, the followers of Krayt were taught blind obedience and absolute loyalty to the Dark Lord of the Sith, instead of the plotting and infighting of previous Sith organizations. Similarly to the Jedi Order, Sith Masters trained a single apprentice. However, after their training was completed, apprentices were required to slay their Masters as a test of their allegiance to the Dark Lord, just like in the past Sith Order acolytes were required to kill someone to whom they were attached or close to seal their transition to the dark side. Masters who deemed their apprentices ready would allow themselves to be killed, as long as it was on Krayt's orders.
The new Sith also mirrored the Jedi in that, while they did rely on passion, hatred and aggression, they did not seek power for themselves alone, but to serve the will of the Dark Lord and impose his order on the galaxy by any means necessary. Instead of reducing the number of Sith to avoid the Order from destroying itself, Krayt altered the Sith philosophy. He claimed he gave power purpose. With the numbers as great as they were before the previous Rule's establishment, it allowed the Sith to match up in numbers with the Jedi. But the ancient spirit of Bane insisted that the use of this rule would be the Order's downfall.
This would eventually prove true, for in Krayt's quest to sustain his life, many of his followers began to question him. Yet, in order to ensure Darth Krayt's vision of a galaxy united under the rule of the Sith, Darth Wyyrlok III killed the Dark Lord as his quest to heal himself had distracted him from his own plans.