Eh, the Sith weren’t really new when the PT came out. The novelizations and the EU had already prominently mentioned/featured them prior to TPM’s release.
I know the novelizations called Vader the “dark lord of the Sith”, but I think its current meaning wasn’t officially revealed until TPM.
Were they in the pre-PT EU too? I admit I don’t know much about the EU except for a few comics and videogames, so please tell more about it.
Wel, the EU did have the Tales of the Jedi comics in the early/mid 90’s. I haven’t read these myself, but from what I’ve gathered part of the series dealt with the Sith War that took place several thousand years before the PT/OT.
Also keep in mind that the early drafts of ANH did mention the "Knights of the Sith" fairly often. The first/rough draft even had several Sith appear throughout the story, and they were essentially just rivals of the Jedi Knights not too different from what ended up in the PT.
The more “evil” and “occult” aspects of them however I think started with the PT, but like I said I haven’t actually read the Tales of the Jedi comics. In the rough draft however they weren’t so much “evil wizards” as they were simply a fascist sect of Jedi-Bendu.