Before the Jedi founded the Republic millennia ago, the Sith ruled the galaxy with an iron fist. The Sith aristocrats enslaved entire worlds to do their cruel bidding, even teaching some of the slaves rudimentary Force techniques to drive the labor of their comrades.
Over time these slave drivers turned against their masters and instead liberated their brothers and sisters from the Sith yoke. In this way the age of the Jedi dawned upon the galaxy.
The first Jedi led a successful rebellion against this ancient Sith imperium. The revolutionary forces pursued the Sith to their final holdout of Korriban.
On Korriban the Sith royal family apparently committed mass suicide. But little do the Jedi know that this action was a ritual to disembody their spirits, to weather the ages and bide their time for the fateful day that the Sith could one day retake the galaxy.
The Sith ghost finds its opportunity in an ambitious Republic Senator named Palpatine.