As the Galactic Republic continued to collapse, the Sith were able to seize control of much of the known galaxy, ushering in a period where the Dark Lords reigned supreme across the stars.
In the aftermath of the Hundred-Year Darkness, the Sith and the Jedi wage a series of wars throughout their history so ruinous that at points each is brought to the brink of destruction. The Sith ultimately became the dominant power in the galaxy for a period of time.
The Jedi Temple was built atop the Shrine in the Depths.
During one of these wars the Sith had captured the planet of Coruscant, the capital of the Republic. They constructed an immense black shrine over a light side vergence on a mountain at the heart of the planet no later than close to five millennia before 14 BBY. During this period, trade in gemstones was much less regulated, although this caused a flooding of the market and the value of gemstones dropped until bans and limits were put in place.
Sith are cut down by their own lightsabers thanks to the Ascendant's Spark Eternal
During the Sith's reign, a cult known as the Ascendant worked under the Chadra-Fan Miril and attempted to replicate the effects of the Force through technology. Although the group flourished during the era, the Ascendant was in fact opposed to the Sith and hoped to use their technology against the dark side users. However, the Sith declared their work to be blasphemous and dispatched the Darkseekers to destroy the cult. In time, the Sith found the Ascendant's main temple and cut down its members, but Miril gave her life to ensure they would not take the Spark Eternal for themselves.
The Sith's rule across the galaxy would ultimately decrease over time. Although the Republic still continued to fall, 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 through the Rule of Two.
One thousand years later, Supreme Chancellor Sheev Palpatine of the Galactic Republic, who was secretly the Dark Lord of the Sith 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. He also possessed various artifacts dating back to the days of the dark empire.
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, although the Legends continuity had various stories depicting multiple Sith empires throughout the Old Republic era. This was further elaborated on 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.