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Star Wars: Knights of the Old Republic Campaign Guide
Darth Revan, Prodigal Knight
... He strikes out for the Rim alone, searching for what he believes is the true Sith menace.
...true Sith menace. Word capitalization may be important here. Using the lowercase letter changes the proper noun. Could the true Sith menace be referring to just another faction? Could the word true be referencing a stronger and more powerful group of Sith? Sith during the Dark Wars were fractured into smaller groups. Could Revan be referring to a more united and stronger faction of Sith in the Outer Rim? There may not be a True Sith species after all.
We may be looking at another group of Sith, but they are more united and stronger. Revan could have evaluated between two Sith threats, and had decided to go after the more powerful faction. We will have to wait and see.