The Sith are a group of Dark Side force users that are enemies to the Jedi. They have been following under the Rule of Two since it was created by Darth Bane, and had before that been a large group nearly as common as the Jedi themselves. Now, only two exist at a time: a master, and an apprentice. In Star Wars Jedi: Fallen Order, Inquisitors are a primary enemy, though they aren't Sith, rather they are hired by Sith, in this case, Darth Vader. They do, however, use the dark side of the force, and in a nearly just as cruel way.
A very long time ago, centuries before the rule of the Empire, the Sith were high in numbers. First came the jedi, though some became corrupted by the dark side, a side of the force more cruel and powerful, making it easy to succumb to. There were constant and multiple large scale wars between the Sith Empire and the Jedi Order. It was much later that Darth Bane created the rule of two to prevent unstoppable chaos and eternal tyranny.
From then on, the known sith that have been master and apprentice are as follows.
- Darth Plagueis (M), Darth Sidious (A)
- Darth Sidious (M), Darth Maul (A)
- Darth Sidious (M), Darth Tyranus [Count Dooku] (A)
- Darth Sidious (M), Darth Vader (A)