*waves because this is self's first post on the forums*
Heya.
Interesting topic.
My take on the 'true Sith'...
The 'true Sith' aren't a sentient species or single being or anything like that, I believe. Sith and Jedi, at the purest form of what they are, what the individual codes and ideals stand for, are the two sides of the Force. Light and dark, both are necessary for balance and both need the other, despite the conflict between each other. The Force is a primal balance of life (think Transcendentalism), its will is for balance alone. This is why Kriea hates it... this will strives for balance, and if human life must be sacrificed in the process, it does not matter. It is destiny. Kriea, though hating the Force with a spite, stems her hatred out of the need for protection of life's ability to have free will.
The two sides of the Force individually don't believe in balance, of course, both sides 'feel' (instinct, mysticism, nothing conscious) their way is the way things should be, and are out to destroy the other.
What the 'true Sith' is is the dark side of the Force in its entirety, the base energy, power, mystic whatever that desires the end of all life. The 'true Jedi' is peace and serenity, no chaos, no death, no suffering just.. simple existance.. life. The Force as a whole must have balance, for life cannot grow and advance without a dark and a light side. Without dark, life would become over-abundant.. without light, nothing could exist.
The Chronicles state that Revan, after the Jedi Civil War, travelled to Malachor V, where he vanished. Revan, IMO, has something deep inside him from Malachor V... that he is the 'Chosen One' of the dark side, of the 'true Sith'. As an evil inside of him, then of course he is the only one who can fight it--no one can help him. Of course, his intentions may be for good or evil, but I believe that when he travels to Malachor V after the JCW, (intending to either destroy the evil that remains or to embrace it), he becomes completely consumed by its evil... similar in a way to Nihilus, (hell, Nihilus might even BE Revan).
He leaves Malachor V and journeys to the areas beyond the Outer Rim (or so we think... as said, he could have been Nihilus. Chronicles only state that he vanished after going to Malachor V). Multiple theories there are for this--one being that he is not fully consumed by the evil and is trying to hold it back... going into exile so as to protect the galaxy (think like Diablo II... bearing Diablo within in order to contain it). Another reason could be that this evil goes into hiding to bid its strength and strike the galaxy at just the right moment. This evil is of course not human, it isn't Revan... nothing that mundane. It is the raw dark side--the true Sith.
It is simply... the will to destroy and consume all life. What Revan goes to fight is the true Sith, contained within him and within Malachor V.
And that was a bit long-winded, sorry.