True Sith

The True Sith are the real decendents of the Lord of the Sith, who died during the Order of the Sith's attempt to overthrow the Galactic Alliance and enslave the Galaxy. They remain true to their Sith tradtions, with one exception; there are now never more than two Sith at any time. The original Sith Species was so infertile that they didn't believe in passing on their heritage through bioloical means. This means, the heir of a Sith was that person's declared heir. In the order of the Sith, the Lord of the Sith picked his heir and that heir became the new lord once the heir grew powerful enough to succeed his master by means of murder. To ensure that no secrets were ever forgotten, all Sith Lords made holocrons.

History
Following the death of the Lord of the Sith, his heir and the apprentice of the heir both went into hiding as they knew the war was lost. As with the rest of the galaxy, they believe Korriban to be generally deserted and only scarcely populated and in no way capable of garnering a military response, and they are entirely unaware of the small number of Sith battleships that made the unexpected decision of fleeing into unknown space.

Being the heir to the Lord of the Sith did not mean being his son or daughter, but rather being his apprentice. His chosen heir, and that is how the line of the Sith was continued.

The heir realized that it was because of the many voices and the infighting that the Order of the Sith had crumbled duing it's attempted overtake of the known Galaxy, which could have succeeded, had they remained in control. The heir instated the Rule of Two, stating that there could never be more than two True Sith at any one time. A master, and an apprentice. A lord and an heir.

The Sith succeeded eachother by murder, and as such, the order continued to live on, how ever ever since the death of the orginial heir's apprentice, there has been no Lord of the Sith who was also a Sith by race.

At some point, the Lord of the Sith came to be a great force behind The Free-trade Alliance and their seperatist movement.

How or even if Revan is connected to the same Lord of the Sith who was behind subtly manipulating the Free Trade Alliance against the Republic is unknown, but so is the existence of the True Sith for that matter.