Sith Empire (Ancient)

The Ancient Sith Empire was formed by Darth Mobius and his apprentice Darth Vexus over 27 thousand years ago. They once ruled the galaxy peacefully until Darth Kudar sealed Mobius and Vexus inside a temple on the Icy world of Ilum, and ruling the galaxy himself, however, Mobius and Vexus escaped only a few hours later and placed a curse on Kudar, slowly and painfully killing him over a period of a few decades. The Galaxy then became lawless and almost destroyed itself until the Galactic Republic and the Jedi Order formed to take the Sith's place, while the Dark Sith Empire became powerful and evil dark-side terrorists, planning to take the galaxy over for themselves.

Return of the Sith
The Sith Empire started to recover its ancient glory when Commander Colt, a Republic Clone Commander discovered Mobius' Planet-sized Dreadnought, the Excalibur, and negotiated with him to convince Mobius to side with the Republic. Mobius saw this as a chance to restore what the Sith Empire used to be, and accepted his offer.