Sith

Sith are the most powerful and prominent Dark Side users in Star Wars. They draw their power from anger, hatred, and the Dark Side of the Force. The term "Sith" was originally attributed to a species strong in the Dark Side, but later the name broadened to refer to an Order of Dark Side users.

Origins
There was an ancient Sith that lived on Korriban who had a strong connection to the Dark Side and were a war-like species.

Hybridization
A group of fallen/Dark Jedi encountered the Sith Species upon being exiled to the Outer Rim after the Great Schisms with the Jedi Order. They intermarried with the Sith species, having offspring that were strong in the Force and achieved mastery over the Sith species. Some examples were Naga Sadow and Marka Ragnos.

Contemporary Meaning
The term Sith eventually became associated with anyone who was part of the Sith Order or subscribed to Sith teachings, regardless of their genetic lineage

"Darth" Title
Revan and Malak were believed to be the first Sith to adopt the Darth Title which they adopted from a Rakatan term for Dark Lord.

The Banite Sith Order and the "Rule of Two"
Note: The Banite Sith are the ones we see in the 6 Star Wars Episodic Films

Formed by Darth Bane circa 1000 BBY, after the collapse of the Sith Brotherhood, the Banite Sith Order focused on a trim idea of the Sith, that there was only one Master and one Apprentice. It was the Master's job to hold knowledge power and the students to crave it. For almost 1000 years they lay undetected while the planned revenge on the Jedi. The Jedi, in fact, believed the Sith were extinct before they revealed themselves when Sith Apprentice Darth Maul engaged Jedi Master Qui-Gon Jinn in lightsaber combat on Tatooine.

The last two Sith of the Banite order were Darth Sidious and Darth Vader, poetically it was Vader who turned on Sidious, who both died during the Battle of Endor.

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