Actually, The Emperor had a number of Dark Jedi under him. At one point in fact he had one of his Hands (y'know, those assassin guys) cloned into an contingent of Darkside soldiers, until they went rogue and Vader had to kill them all. So both sides are right, in a way.
__________________ Minsc Quotes
-"Go for the eyes Boo, GO FOR THE EYES!! RrraaaAAGHGHH!!! "
-"Fool me once, shame on you; fool me twice, watch it, I'm huge."
-"Make way evil! I'm armed to the teeth and packing a hamster! "
-"EVIL, MEET MY SWORD! SWORD, MEEEEET EVIIIL!!!! "
-"After frolicking in a bush that we now consider to be of suspicious nature, both Boo and I have contracted the Calimshite Itch in rather... private places. A salve would be most joyously anticipated! " (Boo is Minsc's 'animal companion'/God).
__________________ "For Fëanor was made the mightiest in all parts of body and mind: in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike: of all the Children of Ilúvatar, and a bright flame was in him.
You know the history expanded upon in the Phantom Menace novelization - it was the great number of Sith users a thousand years earlier, that help seal their fate against the Jedi. They fought & killed amongst themselves, because the dark side makes you greedy for power, so they helped the Jedi in wiping themselves out. The surviving Sith made a pledge to always keep their numbers small after that.
__________________
Right! Now, I'm on a mission!...If I can just remember...What?! I've got no time to talk - I'M ON A MISSION! And...what planet am I on again??
She's mentioned in the Clone Wars cartoon as well as the recent Clone Wars movie and weekly series. All of those are upper canon, as the George overwatches their production/storyline very closely. So, yes she is a canon character.
They're not G-canon, but they are T-canon, hence upper canon. And they're certainly a higher level of canon than the EU books. Although, with that being said... nothing is absolute.
Getting back on subject a bit . . . .I did liked the Killing of the Youngling's at the hand's of Darth Skywalker. Mabie 'Liked' isn't the right word. But I thought it showed just how murderous and how far Skywalker was willing to go to sell his soul to the Dark Emperor. The story of Faust indeed.