We take what we desire because we can. We can because we have power. We have power because we are Sith.
—Sith proverb
The Sith Order, commenly known as the Sith, are a sect of force-sensitives who utilize the dark side of the force. The original term "Stih" originates from a red-skinned species of the same name which was heavil connected to the dark side of the force from the planets of Korriban and Zoist. The species was alter conquered and enslaved by the Exiles, a group of Dark Jedi who were exiled from the Jedi Order after challenging them over the usage of the force leading to the Hundred-Year Darkness whic lead to their defeat and exile. They then fled to Korriban and began interbreading and mixing of alien and human cultures with the Sith species and eventually founded the Sith Order in opposition to the Jedi and the Sith would no longer be identified by race, but rather by philosophy. The new sect was lead by a dark lord and would lead the Sith to the ultimate goal of destroying the Jedi and usurping power in the galaxy.
The Sith would later form an empire and go to war against the Jedi and the Galactic Republic in what would become the Great Hyperspace War which lead to their defeat. Centuries later however, the remnants of the Sith and their fallen empire escaped and would establish a new one on Dromund Kass and would go to war once again and this time, were far more successful than before. The Jedi and Sith would clash numerous times throughout the Old Republic era with the Jedi fighting alongside in defense of the Republic against the Sith and their various different empires. The Jedi and Sith would then clash in the New Sith Wars and after the Seventh Battle of Ruusan, the Sith were nearly destroyed and thought to be extinct following the war's end and the conclusion of the Old Republic era. In reality however, Darth Bane managed to survive and would continue the Sith Order through his philosophy of the Rule of Two which he developed from the teachings of Darth Revan. There, he would form the Order of the Sith Lords and would keep the Sith strictly down to two members which ensured the order's survival and secrecy for thousands of years until the declining years of the Republic when they decided to strike.
The Sith, now lead by Darth Sidious, would orchestrate the Clone Wars and the Republic's demise by commisioning the creation of the Clone Army and allowing the formation of the Confederacy of Independent Systems and taking over their leadership along with creating their vast droid armies to invade the Republic and instigate a galaxy-wide war wich eventually saw the end of the Republic and the systematic destruction of the Jedi Order via Order 66. Afterwards, the Galactic Empire was formed and the Sith now regained control over the entire galaxy and used the empire to enforce their reign and would try to track down and eliminate any survivng Jedi and snuff out any spark of rebellion.