Originally posted by Lord Lucien
Overused and under-explained. Without evoking the EU, I still have no clue what a Sith actually is, aside from a Dark Sider who hates Jedi.
That line, right there, is why it really infuriates me when people badmouth the EU (or "Legends" if you must call it that).
The EU is what MADE Star Wars. It's what provided depth to everything. Characters, places, concepts etc. If you went by the movies only there's practically nothing.
For example, as you yourself pointed out, everything about the Sith, their origins, motives, nature and such, all of that comes from the EU.
Heck, the word "Sith" isn't even mentioned in the OT.