Star Wars: Unseen Forces
The Sith now commonly refers to the dark side force users in opposition to the Jedi. At present such knowledge is exceedingly rare outside of Jedi records, and even though difficult to locate. Between the Rule Of Two which kept them secret and hidden, and the latter Jedi tendency to cover up and conceal anything related to the Sith, knowledge of their existence and powers is largely absent from the galaxy.
Fortunately the recovered data crystals did not suffer the same purge found elsewhere. These detail a limited but useful set of information on the Sith.
The Sith were an order of force users that primarily used the dark side. They often came into conflict with the Jedi. Though once numerous at some point in time a 'Rule of Two' was put in to place limiting the number to a master and apprentice at any one time. If the records are correct they have been behind numerous wars and conflicts in a bid to control the galaxy. As of the time of the data crystals creation they are believed extinct.
The Sith have in a manner of speaking two histories. That of the species and that of the order of force users, which becomes connected at a certain point. The species itself died out eventually.
The order has its roots in the Jedi. The first of what would come to be called Sith were Jedi who experimented with altering and creating life through the force. The rest of the order condemned and banned the practice leading to a hundred year war where those who had been involved with the experiments were defeated and exiled into unknown space.
Eventually they found Korriban where they learned much of the dark side and eventually enslaved the local Sith species. From there they would move their capital to Ziost., and eventually to Dromond Kaas, with the capital shifting with the rise and fall of empires.
The Sith were a relatively unsophisticated species who were nonetheless powerful in the use and manipulation of the dark side of the force. They were enslaved by the Jedi outcasts when they arrived on Korriban. There were three subraces of the Sith, per their species caste system. The Massassi (Warrior), Kissai(Priest), and Zuguruk(Engineer). It is unclear if the differences in these subraces was apparent prior to their interbreeding with humans, but records show the three as having slight differences in physiology.
Over time despite the humans being vastly outnumbered by their Sith servants, the ability for the two to successfully breed resulted in the human side of the resulting DNA proving dominant. Eventually the Sith were bred out of existence. Still, the vast majority of the humans in past Sith empires had Sith DNA. At one point in time the more 'pure' a person was the more respected they might be, the inherent racism continued in the future with the various empires moving from being pro-sith to pro-human. No recent studies have been performed per the data crystals to suggest what portion of humanity now has Sith blood.
The last recorded Sith per the data crystals was over a thousand years prior, they are believed extinct.
The Sith utilized a number of unique tachings and practices in ancient times, most of which were lost and forgotten by latter day Sith or deemed unworthy of their pursuit.
Sith alchemy, sometimes called force alchemy, was a blanket term for Sith sciences that used the force to permanently alter objects, people or things. Sioth Swords were a prime example of the ability to strengthen inanimate objects, though the more sinister and dangerous was arguably the ability to genetically modify creatures.
Sith amulets were focusing crystals used by the ancient Sith to enhance their power. Each had its own focus and abilities and their strength was tied to that of the user to a degree. Academy notes suggest these amplified rather than granting a flat increase as a result. Known examples include amulets that could heal, project force images, protect against lightsabers, and more. They seem to share a common ability, or at least types were common enough that allowed the Sith wearing them to project pure dark side energy capable of killing both the living and even force spirits.
Sith artifacts were various creations via the dark side of the force, often through Sith alchemy. These included the Sith amulets, but also other creations such as Sith spellbooks, Sith Swords, and more. Highly treasured by users of the dark side, they are exceedingly rare in modern times.
Sith sorcery was a unique application of the force that also utilized arcane phrases, gestures, and artifacts to unleash the raw destructive power of the darkside, as well as other more subtle effects. Its use fell out of knowledge or use long ago. The knowledge the data crystals possess on it are largely thanks to the Sith Yevrah was able to revive and convince to become his disciple.
It would seem only those wholly given to the dark side can make use of Sith sorcery, and it was highly destructive to the user as well as the target. Study on it was purely academic in nature as none of Yevrah's students could make use of it, nor could Yevrah himself.