The Rule of Two created a powerful dynamic by limiting the Sith to only one master and one apprentice at a time, although not all Sith followed the decree as some took apprentices of their own while still serving a master.
Always two there are
Following the infighting among the Sith, the surviving Sith Lord Darth Bane created the Rule of Two, stating that there shall only be two; a master to embody the power and an apprentice to crave it.
The ultimate villain of the Star Wars saga, Palpatine was also known as Darth Sidious and was perhaps the greatest Sith Lord in galactic history. Unsurprisingly, he trained many proteges, both officially within Darth Bane's Sith Rule of Two and unofficially.
Sidious broke the Rule of Two at least twice: once by training Darth Maul while still under the tutelage of Darth Plagueis; and secondly, by taking on two apprentices at once, and in the form of two Jedi: Count Dooku, the former Padawan of the legendary Jedi Master Yoda, dubbed Darth Tyranus, and the Fosh Jedi Vergere.
Not anymore, but in Legends, before Disney declared it to not be canon, Darth Vader had 2 apprentices. In The Force Unleashed and The Force Unleashed II, he trained both Starkiller and Galen Marek (not the same person, the latter was a clone of the former).
Galen Marek (pronounced /ɡeɪlən ˈmæɹɛk/), codenamed Starkiller, was a human male Force-sensitive who served as the personal assassin of the Sith Lord Darth Vader, who trained Starkiller as his secret apprentice—but came to the light side shortly before his death.
The Rule of Two created a powerful dynamic by limiting the Sith to only one master and one apprentice at a time, although not all Sith followed the decree as some took apprentices of their own while still serving a master.
Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth. Instead, he was just a chaotic mess of the dark side (until he eventually found his way back to the light).
The individual was first mentioned, albeit indirectly, in Star Wars: The Force Awakens: The Visual Dictionary that Kylo Ren was Snoke's most gifted apprentice, implying he had trained more than one.
The time period of when Plagueis came to possess his Sith apprentice was unknown, but the Dark Lord chose Sheev Palpatine, a human from Naboo, as his apprentice.
By far the most iconic Sith Lord and warrior is Darth Vader (or Anakin Skywalker). He, through his extensive injuries and being trapped in a technologically-advanced suit to survive, stayed strong with the force.
1. Darth Sidious (Star Wars: Episode I – The Phantom Menace) Truly, the most powerful Sith Lord was Darth Sidious, better known in his public persona of Chancellor (later Emperor) Palpatine.
Darth Shain, headmaster of the Korriban Temple, took notice and convened with the Dark Councilor and Dark Lord of the Sith Darth Shane McMahon, who decided to confer the title of Sith Lord to the young apprentice at the age of 11, making him the youngest Sith Lord in the history of the Sith.
History. A dark side warrior with a mysterious past, Kylo Ren was neither Jedi nor Sith, but a product of both sides' teachings. Once an apprentice of Luke Skywalker's, he killed his fellow students and drove Skywalker into exile, becoming a First Order warlord and servant of Supreme Leader Snoke.
Sith apprentices were analogous to Jedi Padawans, and the relationship between Sith Master and apprentice was the analog to Jedi Knight and Padawan.
Lucas fleshed out the Emperor in the prequel films. According to Lucas, Palpatine's role in The Phantom Menace is to explain "how Anakin Skywalker [later Vader] came to be Palpatine's apprentice" and the events that led to his rise to power. The film's novelization is the first time he is called Darth Sidious.
How Palpatine Saw Darth Maul. Maul was Palpatine's apprentice during his silent rise to power, existing as his strong right hand and resenting the Jedi for never seeking him out as a child and sparing him Palpatine's 'training' as a Sith.
Dathan had a relationship with Miramir, who gave birth to their daughter, Rey. In addition to rejecting Sidious, Dathan married a woman named Miramir, whom he fell in love with. Together, they conceived a child, their daughter Rey, who was born in 15 ABY.
Darth Jar Jar, known to most as Jar Jar Binks, was a Gungan Sith Lord. He survived the Battle of Naboo and became a Representative of Naboo, when he helped to give Darth Sidious emergency powers. After the fall of the Galactic Empire, he forced a small alien named Snoke under his evil wing.
Dooku voluntarily left the light side behind and became Darth Sidious' dark side disciple, taking the secret name Darth Tyranus and leading the Separatist army.
Many Sith Lords chose to add "Darth" to their name, so much so that the word is considered synonymous with the dark side of the Force in some circles. It was also taken to signify giving up one's old life. Such examples are Anakin Skywalker becoming Darth Vader, or Jacen Solo becoming Darth Caedus.
While many Sith Lords are mentioned, only six are detailed in canon: Bane, Plagueis, Sidious, Maul, Tyranus, and Vader. In addition to these Sith Lords, there are a number of other fallen Jedi, apprentices, and dark Force users.
After the Sith were decimated by the Jedi Knights of a thousand years ago, Bane enacted the Sith rule of two: there would be only two active Sith at one time -- a Dark Lord to embody the power, and an apprentice to crave it.
Darth Zannah was a Dark Lord of the Sith and the Sith apprentice of Darth Bane. Zannah was trained to carry Bane's teachings and the Rule of Two forward after a war between the Sith and Jedi Orders.