The recent finale of Star Wars Rebels had everything. Crazy former Sith? Check. Blinded master? Check. Temples and an epic duel between Ahsoka and Darth Vader? Triple check. But a dominant theme throughout the episode was the Sith holocron, an ancient artifact used to keep the knowledge and sometimes even the spirits of Sith. At the very end of the episode, Ezra unlocks the holocron and goes all Sith evil eyes on us, but we never get to know whose holocron it is. But I believe that Knights of the Old Republic 2 the Sith Lords unveiled the true identity of the mystery Sith long ago. Here is my evidence for this fan theory and be advised, at present it's pure conjecture.
1) The Sith is female.
The voice that comes out of the holocron is distinctly female. It is very easy to spot this when she's talking to Ezra.
2) The location of the holocron is Malachor.
Malachor 5 is the system in which a certain dark lady of the Sith started a Sith academy known as Trayus Academy(or restarted if you prefer following Revan's discovery in kotor 1).
3) The holocron says something interesting.
The holocron says, "Knowledge is power". A seemingly innocuous quote, but if you look deeper, it has meaning because only someone steeped in the arcane arts and searching for knowledge would say that.
The answer? Darth Traya!
Darth Traya or Kreia as we know was an ancient dark lady of the Sith who was a member of the Sith Triumvirate in the time of the old republic. She was originally a historian for the jedi which explains her scholarly origins. She was based on the Trayus Academy on Malachor V and she was the one who used the exile as a weapon in kotor 2. Also, she was destroyed by the exile. Traya will be an excellent addition to rebels after Bane was shown in clone wars and I love the fact that they are exploring so much of the legends mythos after it being declared non-cannon by Disney.