Many centuries before the time of the tale of the sky walkers or that of the struggle between the Empire and the Rebellion, there was another war fought to determine the fate of the galaxy forged by the Sith Empire and the Jedi Order to determine the fate of the galaxy.
Out of the conflict, the Sith were supposedly destroyed from the face of the galaxy, however one survivor remained and formed a Rule of Two and A Grand Plan of Revenge Against the Jedi and the Republic.