The Doctrine of the Dyad preceded the Sith Order's Rule of Two. The Doctrine of the Dyad revolved around the concept of a Force dyad. This dyad represented a pairing of two beings, each Force-sensitive, connected by a Force-bond so profound it could not be broken. This doctrine came before the establishment of the Rule of Two. The Rule of Two was a principle established by Darth Bane that restricted the Sith Order to only two Sith Lords at any time: one Sith Master and one Sith apprentice. Despite the Rule of Two preserving the Sith for a millennium, Darth Sidious, the Dark Lord of the Sith, considered it a "pale imitation" when compared to the Doctrine of the Dyad.
The Doctrine of the Dyad was inscribed on the walls within the Sith Citadel, situated on the planet Exegol. These inscriptions served to remind the Sith Order, along with the Sith Eternal cult, of the doctrine's significance. For a thousand years under the Rule of Two, the Sith relentlessly pursued the creation of a dyad through sheer force of will, but failed. Darth Plagueis was unsuccessful in forming a dyad with his apprentice, Darth Sidious. Sidious, in turn, was equally unsuccessful in creating a dyad with his own apprentice, Darth Vader. The Sith Eternal, a group of Sith cultists devoted to the Sith religion, regarded the dyad as the Sith's future, the key to unlocking the dark side of the Force's complete potential. Their lore included a prophecy that foretold the eventual emergence of a Force dyad.
Many generations elapsed before a dyad manifested in Ben Solo, Darth Vader's grandson, and Rey, Darth Sidious' granddaughter, during the New Republic Era. Their extraordinary Force connection forged a bond that enabled Solo and Rey to communicate across both time and space. During the Battle of Exegol, Sidious became aware of their dyad nature and proceeded to drained their combined life force to rejuvenate his weakened clone body. Following Sidious's demise, Solo gave his own life force to Rey, bringing her back to life but at the cost of his own.
The Doctrine of the Dyad was initially brought up in Star Wars: The Rise of Skywalker: Expanded Edition, the 2020 novelization inspired by the film of the same title. It was formally identified as such in the 2021 reference book titled Star Wars: The Secrets of the Sith.
Within the Star Wars Legends continuity, the 2011 novel Knight Errant, penned by John Jackson Miller, features the Dyarchy Sith principality. This principality was governed through a comparable Force bond that existed between the Sith twins Quillan and Dromika during the Republic Dark Age. They were characterized as "separated in body, but conjoined through the Force" and "one being, that no power in science or Sith alchemy could separate."