Era of the Sith

During the Old Republic's decline, the Sith were able to seize control of much of the known galaxy, ushering in a brief period where the dark lords reigned supreme across the stars.

History
Many years after the formation of the Sith in the aftermath of the Hundred-Year Darkness, the Order returned to Republic space to do both war with it and the Jedi, their ancient enemies. For millennia both factions fought against one another, and periods of constant warfare ultimately strained the Old Republic's resources. With the steady collapse of the Republic at hand, the Sith Empire ultimately became the dominant power in the galaxy for a short, but oppressive, period of time.

The Sith's rule across the galaxy would ultimately decrease over time. Although the Republic still continued to fall, then entering a chaotic period known as the Dark age, the Jedi and remnants of the Old Republic would have one final war against the Sith, bringing an end to their despotic reign once and for all, although the Order survived in secret as the Rule of Two.

One thousand years later, Chancellor Palpatine of the Galactic Republic, who was secretly the dark lord Darth Sidious, transformed his government into the Galactic Empire, declaring himself emperor in the process. He was an admirer of the Sith Empire and the Sith Lords who ruled it and wished to usher in a renewed period of its rule, also possessing various artifacts dating back to the days of the dark empire.

Behind the scenes
A period of a Sith controlled galaxy is first mentioned by Sheev Palpatine in a scene from the 2005 film Star Wars: Episode III Revenge of the Sith. This time was never specified, as in Legends, which was seen as the official Star Wars canon at the time of the movie's release, had various stories depicting multiple Sith empires throughout the Old Republic era. This was ultimately cleared up after Book of Sith: Secrets from the Dark Side was published in 2012, when, in a segment "written by" Darth Sidious, it was stated that he admired the dark lords Malgus and Vitiate of the Second Sith Empire and modeled his vision of the Galactic Empire after the civilization, which was at its height during the Cold War.

Although none of this is canon any longer, it has still been established that Palpatine admired the Sith Empire of old and wished to restore it to the glory that it possessed when it was at its height.

Appearances

 * Star Wars: Episode III Revenge of the Sith
 * Darth Maul, Part II
 * Aftermath: Empire's End