The Nightsisters of the Star Wars Legends continuity may not be as powerful as the Sith, but they have at least one dark side power that even the Dark Lords can't access. In the now-bygone Legends continuity, the Witches of Dathomir consisted of powerful Force-sensitive human women (as opposed to the canon-exclusive Dathomirian species) with their own unique Force-using traditions. One such group, known as the Nightsisters, used the dark side of the Force in inventive and potent ways against their enemies.
Thanks to a retcon, the very first Nightsister shown in Star Wars Legends is Charal, who abandoned her sisters and fled Dathomir only to end up stranded on the Forest Moon of Endor, where she became a high-ranking member of a gang of Sanyassan Marauders. In Charal’s live-action debut in Ewoks: The Battle for Endor and her appearance in Star Wars: Ewoks – Shadows of Endor, by Zack Giallongo, she has the ability to transform into a raven. No Sith Lord is shown with such a power, perhaps because it involves becoming less immediately powerful for long-term benefit.
