Saga When did "the Sith" return to Star Wars?

The early EU (largely the Dark Empire trilogy) made a point of keeping Palps separate to, yet associated with the Sith. Vader's story was very much off-limits for the EU, but in all official sources, bar the films themselves, he was always "The Dark Lord of the Sith".

The general impression I got prior to the PT was that Anakin/Vader fell in with some sort of anti-Jedi cult called 'the Sith' who were wiped out during whatever conflicts led to the fall of the Republic, and Palps (who was some sort of non-affiliated Dark Side sorcerer) took him under his wing in order to consolidate his own power.

It's sort of an upside-down version of the early drafts' accounts of the Sith, in which the Sith were once a band of mercenaries trained by a rogue Jedi named Darklighter in the ways of the Force - then grew to become a dangerous organisation which the Emperor himself saw fit to employ, somewhat desperately. This did change by the very first draft of ESB, in which Vader is somewhat in fear of the Emperor (who has knowledge of the Force), but prior to that, you get the feeling in the drafts and novelisation that the Empire might as well have been bringing the Joker in to take care of their dirty work:

 

How? He still did what no other Sith Lord had done. He got the Republic to hand him the keys to the kingdom and eliminated the Jedi to just two members. And more, he turned the Chosen One to the dark side. He will go down in the annuls of Sithdom as the man who did the most damage.

Ah, but that was important because it allowed the Sith to be destroyed in ROTJ. Thus allowing Anakin to fulfill his destiny.

 

but now there's a sequel trilogy coming.

 

Which doesn't mean that the Sith are in the films. We don't know what the threat is, or where it comes from. Or if there is a Sith, it might be one of Luke's students turning to the dark side on their own, just as it happened long ago. That's why Lucas made a point of showing that a Jedi can go bad with Dooku before showing Anakin's downfall.