Sith Purebloods are the original Sith. Sith isn't just a faction, but was once a species. A civilization. The homeworld being Korriban and their affinity for the Dark Side is what earned them a near extinction. Near extinction by way of a few rogue Sith Purebloods still roaming the galaxy during The Old Republic Era. Sith Purebloods are a barbaric and warmongering people, and are often hostile to anyone they speak with out of principle.