A Sith Master was a rank in the Sith Order, reserved for a Sith Lord who instructed a Sith apprentice in the arts of the dark side of the Force. The role of a Sith Master was to pass their knowledge to the student. By tradition, the apprentice had to kill their Sith Master in order to complete their training, or the master could train another student to kill and then replace their outgoing student.
Under the Rule of Two established by Darth Bane, the Order's ranks were limited to two Sith Lords—a Sith Master and an apprentice—at any given time. After a millennium of hiding from the Jedi Order, the Sith returned to power in the galaxy through the rise of Darth Sidious, the Dark Lord of the Sith who killed his Sith Master, Darth Plagueis, and mentored the Sith apprentices Darth Maul, Darth Tyranus, and Darth Vader.
In the Sith Order, a Sith Master was the title of a Sith Lord who trained a Sith apprentice in the ways of the dark side of the Force. It was the duty of a Sith Master to learn all that there was to know about the Force, and then to pass this knowledge to their Sith apprentice.
Darth Sidious was the Sith Master who trained several apprentices during his reign as the Dark Lord of the Sith.
Approximately a millennium before the Invasion of Naboo, the Sith's greed for power led to infighting, which caused them to be defeated and nearly destroyed by the Jedi. As a consequence, the last survivor, Darth Bane, established the Rule of Two, which restructured the sect so that there could only be two Sith at a time, a master and an apprentice. In the last decades of the Galactic Republic, the Sith Master Darth Plagueis tutored an apprentice in the ways of the Sith. After betraying and murdering Plagueis, Darth Sidious subsequently took over his master's role as the Sith master, and managed to reach the highest echelons of democractic power before anointing himself Galactic Emperor.
Darth Tyranus, although still apprenticed to Sidious, trained Asajj Ventress in the ways of the dark side.
Darth Tyranus trained Asajj Ventress as his apprentice during the Clone Wars, though he was still apprenticed to Darth Sidious. As Ventress grew in power, Sidious grew suspicious of Tyranus' intentions. He ordered his apprentice to kill the Nightsister assassin, and Tyranus complied despite regarding Ventress as his most reliable asset. Tyranus then sought to replace Ventress with Savage Opress, intending to overthrow his Sith Master after training the Dathomirian former Nightbrother in the ways of the dark side.
Darth Vader rebelled against his Sith Master, saving his son and dying as Anakin Skywalker.
Sidious eventually betrayed Tyranus by ordering Anakin Skywalker to kill him. Tyranus was then replaced by Skywalker, who pledged fealty to his new Sith Master as Darth Vader. Like his predecessor, Vader was an apprentice who mentored Force-sensitive students despite not holding the rank of Sith Master. His apprentices belonged to a cadre of fallen Jedi-turned-Jedi hunters known as the Inquisitorius. Sidious eventually sought to betray and replace Vader, by becoming the Sith Master of his apprentice's son, Luke Skywalker. However, he was killed by a redeemed Anakin Skywalker, after Luke pledged himself as a Jedi like his father before him.
Kylo Ren, the grandson of Vader, was lured to the dark side through the machinations of a revived Sidious and his Strand-Cast clone, Supreme Leader Snoke of the First Order. Although Snoke directly oversaw Ren's training, Sidious considered himself to be Ren's true master. By using Snoke as a proxy mentor for Ren, Sidious managed to circumvent a dark side tradition in which the apprentice completed his training by killing the master who trained him, as Ren killed Snoke rather than Sidious.
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