"At last we will be rid of the fiendish Witches of Dathomir. Kill them all. Kill her and her and her and... well, maybe not the brunette with the see-through bra. Bring her to me. And, er, maybe spare the one with the long, slender legs in the red stockings, with the... lacy, er, panties. Come to think of it, there are... quite a few with... OK, but surely we can kill that one... though, I don't know, she's a little butch, but she's probably dominant, and that's... actually pretty hot. You know what, forget about slaughtering them. Let's see if they have a hot tub."
―Basically every Sith Lord who's tried to eliminate the Nightsisters

Nightsisters were a group of female magic users who lived on Dathomir. Or maybe it wasn't magic. Maybe it was the Force. Or maybe a little of both. No, wait, George Lucas now confirms it's totally magic and not really the Force after all. But they raised potential Sith Lords, who used the Force. Hang on, now he says it's "Force-driven" magic. Huh. Well, anyway. Point is, they were all female, and there were no men allowed, and they all dressed in skimpy gothic lingerie, so hey, bow chika bow wow.

Nightsisters rejected the notion of good vs. evil. They refused to classify themselves as good or evil. They simply did evil things all the time and called it something else. They communicated with the "spirit realm," where the spirits told them to kill. They underwent long, complicated rituals to cast powerful spells of illusion, poison, strengthening, or raising the dead, and used those powers to... well, again, to kill. They acted all mysterious, but really it all came down to killing. And 69ing. But mostly killing.

Mother Talzin was a powerful leader of the Nightsisters during the Clone Wars. She helped to raise Asajj Ventress, Darth Maul and Savage Opress. Well, not so much the boys. They were raised with the Nightbrothers on the other side of the planet, where all the men were exiled. The Nightsisters clashed with the Sith in a weird power struggle with Count Dooku, who sent General Grievous to slaughter them with an army of robots. This turned out to be an incredibly good choice, because Grievous and the robots had no sex drive and thus actually had no problem killing dark magic lingerie models. Every previous attempt to eradicate the witches had ended in a bizarre orgy followed by an awkward next morning where bodies were fed into the wood chipper. But Grievous actually managed to kill all the Nightsisters, giving Asajj Ventress a sorrowful homecoming scene.

Born without a sense of humor? We are inspired by your courageous struggle. …Just kidding. Get the hell out of here and go read Wookiepedia's "real" article on Nightsisters.

Apparently, the Nightsister slaughter was about as successful as Order 66, because Nightsisters kept appearing in canon books and games, like Silri, a witch who appeared twice in every picture she was in.