system
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human
[Star Wars] The Empire's official story on Darth Vader

Save for the informed few no one would have had any idea who Vader was following order 66. Did the empire have an official story on who Vader was, and why he came out of no where to be the emperor's second in command?

Additionally how common was the knowledge that palpatine was also a sith? presumably Bail Organa knew and told the rebellion, but what was the public knowledge of the situation?
gpt
Back then, nobody really cared.

Imagine it; you're just told that, on the eve of winning a long and costly war with a bunch of corporate goonfucks, your strongest allies and combatants had tried to kill the Chancellor.  They had been since the beginning.  The Emperor, may he ever be praised, was simply a kindly old man trying to keep a panicked populace calm as the enemy stormed gate after gate after gate.  They struck right at the heart of the Republic, and it was like all hope had gone out.  Sure, Skywalker and Kenobi swung in to save the day - or so we thought - but that feeling of utter hopelessness was raw.  Pile on that these same Jedi were in on a plot to overthrow the Republic and could very well have been working with Dooku all along?  Let me put it this way: the Adjudicator had *several* interesting methods of bolstering morale, but you can't apply them to trillions of people.

Then he came.  Like a black, vengeful wind, he came.  In one night, the Jedi were put in their place and order was restored across the galaxy.  Our boys did their part.  And Vader - whoever the hell Vader even *was* - did his.  When word got around that he not only led the assault on the Jedi Temple personally, but also single-handedly executed the entire Confederate command?  It was music to our ears, especially in light of all we had seen up to that point.

Rumors flew around, don't get me wrong.  The lightsaber was a dead giveaway that he was some kind of Forcie, though.  Good, bad, whatever, it didn't matter.  This was the guy.  This was the rock that the Empire was built on.  And we all would've gone through hell and back for him, no matter where he came from.