The Jedi Order and Sith Order were bitter adversaries for millennia. Eventually, the Sith’s greed for power caused them to turn on themselves, which allowed the Jedi to defeat and decimate the weakened Sith a millennium before the Clone Wars. However, Darth Bane emerged as the sole surviving Sith Lord. Realizing that the Sith’s greed led to their destruction, Bane created the Rule of Two, to allow the Sith to operate in secret and prevent them from being destroyed by their own greed again. Bane also created the rule so that the Sith could eventually gain enough power to take revenge on the Jedi.
Bane developed the philosophy after his personal experiences, by studying ancient Sith documents and to succeed the older, purer doctrine of the dyad in the Force, a unique Force-bond strong as life itself between two Force-sensitive individuals. In comparison to the dyad, the Rule of Two was a pale imitation, an unworthy but necessary successor.
Under the Rule of Two, a Sith apprentice had to kill the Sith Master in order to ascend the dark side of the Force, ensuring that the Sith grew more powerful and cunning with each generation. On the other hand, the master could train another student, and have them replace and kill the outgoing student. As a result, the relationship between the master and the apprentice lacked trust, as both constantly searched for signs of weakness in the other.
Bane took an apprentice of his own, Darth Zannah, and passed on his knowledge while establishing the new Sith way of pursuing power and revenge against the Jedi through manipulation and subterfuge. The Jedi became aware of Bane’s philosophy and Bane was killed. Although historical records weren’t accurate, it was considered possible that Bane was killed by his own apprentice. In either case, the Jedi went on to falsely believe that they had destroyed the Sith forever. However, Bane’s plans continued through his apprentice, allowing the Sith to survive secretly for years while planning their revenge against the Jedi Order. Rumors, nevertheless, remained that the Sith continued in the shadows. The Rule of Two created a powerful dynamic by limiting the Sith to only one master and one apprentice at a time, although not all Sith followed the rule as some took apprentices of their own while still serving a master.