A Twi'lek Dark Lord of the Sith held the position of Sith Master to the Bith male Darth Tenebrous during the closing centuries of the Galactic Republic. This pair were successors in the lineage of Darth Bane, a previous Sith Lord, and they followed his Rule of Two, which dictated that the Sith could only have one Master and one apprentice. While serving as Master, the Twi'lek created a hole in the Force, which allowed the Jedi Order to sense the dark side for the first time in over eight centuries, since the final battle of the New Sith Wars. Ultimately, Tenebrous killed the Twi'lek, and then continued the Sith imperative by choosing the Muun Hego Damask as his apprentice, and naming him Darth Plagueis.
In the last centuries of the Galactic Republic, a Twi'lek sensitive to the Force became a member of the Order of the Sith Lords, eventually rising to the position of Dark Lord of the Sith. In accordance with the Rule of Two of the Sith Lord Darth Bane, which restricted the Sith to a Master and apprentice, the Twi'lek Sith Lord eventually chose the Bith male Rugess Nome to be his student, and gave him the name Darth Tenebrous. The Twi'lek Sith Master and Tenebrous were committed to furthering the Grand Plan, a Sith goal that was centered around destroying the Jedi Order and the Republic. The pair spent years trying to create a virus designed to target and sever a Jedi's connection to the Force. However, the Master and Tenebrous ended up abandoning this project. At some point during the Twi'lek's time as Sith Master, he created a rift in the Force, which enabled the Jedi Order to detect the dark side of the Force for the first time in the eight centuries following the final battle of the New Sith Wars.
The Sith Master of Darth Tenebrous made the Bith study the Force, something that Tenebrous considered to be simple. Eventually, the Bith came to the conclusion that the Force was a source of energy that he could control in order to achieve his goals. After reading the prophecies in the Journal of the Whills about the Chosen One who would bring balance to the Force, Tenebrous decided to use a version of essence transfer—a Force technique that allows the user to move their consciousness into another body—at the moment of the individual's birth, so that he could become the Chosen One. Sometime between approximately 167 BBY and approximately 124 BBY, when Tenebrous's calculations for his plan were finished, the apprentice killed the Twi'lek, and then went on to choose the Muun Hego Damask as his apprentice, and gave him the name Darth Plagueis.
Plagueis, who eventually succeeded the Sith Master, recognized the Twi'lek's role in advancing the Grand Plan to destroy the Jedi and the Republic, as well as in creating a rift in the Force. The Muun Sith Lord told his apprentice, Darth Sidious, about the Master's and Tenebrous's accomplishments.
The Twi'lek Sith Master was committed to the Sith plan to destroy the Jedi and the Republic. Although the Master and Tenebrous tried to create a virus that would separate the Jedi from the Force, they ultimately gave up on this effort. Tenebrous considered his Master to be simplistic and unsteady.
The Master of Tenebrous was skilled enough to create a rift in the Force that allowed the Jedi Order to sense the presence of the dark side for the first time since the final battle of the New Sith Wars. However, the Twi'lek was not strong enough to defeat Tenebrous, who killed his Master.
The Sith Master of Darth Tenebrous was first mentioned in the short story The Tenebrous Way, which was written by Matthew Stover for Star Wars Insider 130 and released in December 2011. The Sith Master was later mentioned in the novel Darth Plagueis, which was written by James Luceno and published on January 10, 2012. The novel established that Tenebrous's Master was a Twi'lek species.