Originally Answered: Why do the Sith take apprentices? This is the same thing for a King to have a son, an heir! For the Sith this is the same thing, an heir for his knowledge and his "power", the apprentice will do the same thing and become the master, take an apprentice and continue to teach his knowledge, an heir!
While a Padawan often accompanied their Master on whatever missions they partook, Sith Masters often sent their apprentices on missions alone. The Sith apprentice acted as an agent of the Master, doing their bidding and carrying out their will without question.
The decree commanded that only two Sith Lords must exist at any given time: a master to represent the power of the dark side of the Force, and an apprentice to crave it and train under the master and to one day fulfill their role.
The Sith existed to serve the dark side, and Sith Lords cared about their Apprentices if the latter showed they were well-versed in wielding dark-sided power. If not, Masters killed off their Apprentices and sought another, like when Palpatine pursued Anakin Skywalker.
In the same way Silicon Valley entrepreneurs care about those employees or juniors which have strong potential to become entrepreneurs themselves, Sith Masters care about their apprentices.
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.
Darth Plageuis is one of the most famous Sith Lords in the Star Wars Universe. Plageuis was the mentor and master to Darth Sidious, and he was spoken of to Anakin in Star Wars: Revenge of the Sith.
1 Darth Sidious
As well as being a gifted bladesman Sidious' connection to the Dark Side is extremely powerful, granting Sidious the ability to conduct force lightning. Likewise, Sidious took on both Darth Maul and Darth Vader as his apprentice, two of the most powerful Sith and best villains in all of Star Wars.
Bane created the Rule of Two in a way which would prevent the Sith becoming like Kaan's Brotherhood of Darkness; Darth Bane thought the Brotherhood was weak and decided he must destroy it, then rebuild the Sith order as otherwise they were doomed to fail like all Sith before them.
Galen Marek, codenamed Starkiller, was the son of a Jedi fugitive killed by Vader during the Great Jedi Purge. After his father's death, he was trained by Vader since childhood to become the Sith Lord's secret apprentice.
Deities. Light Sith or light-sided Sith were adherents of Sith teachings that rejected the dark side of the Force in favor of the light while retaining their devotion to the Sith as an organization.
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 Maul was the first Sith apprentice of Darth Sidious. Sometime before the Invasion of Naboo, the Sith Lord Darth Sidious was trained by Darth Plagueis.
Sith warriors wore robes designed with a hood which could conceal the wearer's face, keeping them from being identified.
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.
During the 2003 animated micro-series Star Wars: Clone Wars, Count Dooku leads the Separatists from behind the scenes, taking the dark assassin Asajj Ventress (voiced by Grey DeLisle) as his apprentice while training General Grievous in lightsaber combat.
Regardless, Dooku's eyes weren't yellow because he wasn't motivated by anger. He was merely a practical dark side user, so he does not have the usual Sith eye color.
The Sith stayed in hiding for a thousand years, their survival unknown to the Jedi, as they continued their revenge against the Jedi through Bane's Grand Plan. With Sith teachings passed down by Bane's Rule of Two, the Order managed to survive.
Construction. In the construction of a Sith lightsaber, individuals partook in a process known as bleeding. By ways of the Force, they would pour negative emotions, such as rage, hate, fear, and pain into kyber crystals. The process would result in the crystals gaining a distinctive crimson or red hue.
He then orders Ren to bring Rey (Daisy Ridley) to him after she refuses to disclose the map to Luke. After Ren is defeated in a lightsaber duel with Rey, Snoke orders Hux to bring Ren to him to complete his training.
There aren't many Sith Lords in Disney's canon, and only one of them is female—Master Shaa, the First Lady of the Sith.
By far one of the cruelest Sith to ever live, Vitiate was created as an antagonist for the game Star Wars: The Old Republic, in which he was responsible for killing his parents at age ten, and later kickstarting the Mandalore Wars, the Jedi Civil War, and in his quest to remain Emperor of the Sith for all time, the ...
Darth Sidious, also known as the Emperor and Supreme Chancellor Palpatine, is not just the evilest Sith in Star Wars (arguably in both canon and Legends) but the evilest villain overall, deemed so thanks to his irredeemable actions throughout the Skywalker saga and surrounding content.
Truly, the most powerful Sith Lord was Darth Sidious, better known in his public persona of Chancellor (later Emperor) Palpatine. Through cunning and manipulation, Sidious killed his master to claim the mantle of Dark Lord of the Sith.