The dark side of Star Wars
The Sith have searched the galaxy for the teachings of all the various dark-side religions. When the Sith population was much larger before the implementation of the Rule of Two, they dispatched agents known as “Darkseekers” to multiple parts of the galaxy to discover new powers and eliminate any possible threats.
The Brothers of the Ninth Door were unmatched when it came to dark-side illusion-casting.
Author George Mann On the planet Jedha, 350 years before the events of the prequel trilogy, takes place in the epic battle known as “The Battle of Jedha” (which was seen in Rogue One: A Star Wars Story).
Though droids and recording devices may be tricked, this new Force power appears to be psychic in nature and involves an entrance into another person’s psyche. The Brothers of the Ninth Door appear to be able to effortlessly weave their illusions into the brains of even renowned Jedi Masters, making the psychic side of their power all the more remarkable. This allows the Brothers to complete tasks while in disguise and evokes the Force magic practiced by the Night sisters of Dathomir.
Darth Plagueis knew the Sith would covet the brothers’ Force might.
As the Rule of Two reinterpreted the Sith Code, this kind of authority would have been highly sought after by the Sith. The Baneite Sith worked covertly, hiding their activities from the Jedi; therefore, a Force illusion powerful enough to fool a Jedi would have been invaluable to them.
We will never know if the Sith mastered the Force abilities shown by the Brothers of the Ninth Door. In any case, if they had, it would have been a step toward masterminding Palpatine’s Force concealment, which was so strong that it permitted the Dark Lord to utilize the Force in the Jedi Temple without being detected. Perhaps the history of this extraordinary Sith power was only hinted at in Star Wars.