A Sith Master was a title held by a Sith who taught at least one student in the ways of the dark side of the Force. The traditional path required the apprentice to eliminate their teacher before they could train their own student and be promoted to the rank of Master.
Throughout the chronicles of the Sith, the Master also carried the titles of Sith Lord and Dark Lord of the Sith. During the era of Darth Revan's Sith Empire, Sith Masters were considered Sith Lords, but only the most powerful Master reigned as Dark Lord of the Sith. Under Darth Bane's Rule of Two, the core tenet of the Order of the Sith Lords, both the Master and their apprentice jointly held both titles.
The Sith Master's function was the opposite of that of a Jedi Master, a title bestowed upon members of the Jedi Order—followers of the light side of the Force. Similar to the Sith Rule of Two, the Jedi Code explicitly prohibited any master from simultaneously training more than one student.
Across the various iterations of the Sith Order, the Master rank was occupied by Force-sensitive individuals aligned with the dark side who mentored one or more students in the ways of the Sith. During the time of Sith Lord Darth Revan, Sith Masters were forbidden from instructing multiple students at the same time—a policy influenced by Revan's belief that weaker Sith would collaborate to overthrow their outnumbered Master. As an illustration of his own policy, Revan trained only one student, Darth Malak, during his time as the ruling Dark Lord of the Sith.
Approximately three millennia following the civil war between the Jedi Order and Darth Revan's Sith Empire, Darth Bane caused the demise of thousands of Sith during the final stages of the New Sith Wars. The Brotherhood of Darkness, an order of numerous Sith, had left Bane disillusioned with the way that the organization deviated from Sith traditions. Following the complete destruction of the Brotherhood at the Seventh Battle of Ruusan, Bane established the Order of the Sith Lords. Guided by the Rule of Two, only two Sith Lords were permitted to exist at any given time—a Master who embodied the power of the dark side, and a student who desired that power. Bane, who had become inspired by Revan's views on the Master-apprentice relationship, trained only one student as his eventual successor, Darth Zannah. Over the next millennium, the role of Sith Master was passed from teacher to student, typically through mortal combat, as Bane had intended.
The Rule of Two remained in effect during the final century of the Galactic Republic, though it was no longer strictly followed by contemporary Sith Lords. Darth Plagueis, like several Masters before him, hesitated to train an apprentice, only to be replaced through eventual betrayal and murder. Plagueis's own Master, Darth Tenebrous, had secretly trained another student, Darth Venamis, without his apprentice's awareness. Plagueis hoped to end the homicidal tradition of the Rule of Two with his apprentice, Darth Sidious, but was ultimately killed by the ambitious Sith Lord who sought to rule the galaxy as Emperor.
Darth Sidious mentored several students at different points in his life. While still learning the ways of the dark side under Plagueis's instruction, Sidious trained a Dathomirian Zabrak, known as Darth Maul, as a Sith assassin. Maul was succeeded by two former members of the Jedi Order, Darth Tyranus and Darth Vader respectively.
Although Darth Vader never achieved the rank of Sith Master, he secretly instructed his own Sith apprentice in a plan to overthrow the Emperor. Galen Marek, a prodigy of the Force, served the Dark Lord as his personal assassin for years under the codename "Starkiller." Marek's redemption as a Jedi and untimely death forced Vader to clone his former apprentice in the hopes of creating a stronger and more loyal version of the original Starkiller. Out of the numerous test subjects who failed to meet Vader's expectations, including a renegade clone, only one clone completed his trials and became Darth Vader's new apprentice.
The Order of the Sith Lords ended with the death of the final clone of Emperor Palpatine in 11 ABY, but the Rule of Two was preserved by a new Sith Master, the self-proclaimed Dark Lady of the Sith Lumiya. Having received partial Sith instruction under both Sidious and Vader, Lumiya re-established Bane's Order and instructed two students in turn, the former stormtrooper Flint, and a Royal Guardsman named Carnor Jax. Her third and final student, Darth Caedus, achieved Sith Mastery shortly before Lumiya's death in a duel against Luke Skywalker. the grandson of Darth Vader, Caedus anointed the Jedi Tahiri Veila as his apprentice, and thus continued the Rule of Two until his own death in 41 ABY.
Even prior to the collapse of Lumiya's Sith Order, a new policy was established by a Sith Master named Darth Krayt. The Rule of One reopened the Sith ranks to more than two Sith Lords, and thus allowed Krayt's Order to expand with more than several hundred members by 130 ABY. The tradition of Master and apprentice remained, but the revived Sith Order was composed of many Masters, each with their own apprentice, and all ultimately answered directly to the one Dark Lord of the Sith.