In a Galaxy Far Far Away...
There was a Sith that was feared by republic and jedi alike. This Sith was revered as a hero to the citizens of the Empire while to the other Sith they respected there power.
But did not dare try anything against them for fear took over when they remembered what happened to the fool who once tried such things.
Then one day the Sith and the closest people to the Sith disappeared from the Empire leaving everyone in the galaxy in a panic. For the Empire lost it's sharpest sword and shield. While the Republic wondered if or when the Sith would immerge from the shadows and strike them down.
As for the Sith themselves you will have to wait and see for yourselves what happened.