Black Speech: The Nightsisters use a rather creepy sounding language for their enchantments.
Both sexes augment their natural skin coloration with ritual tattoos, but Nightbrothers get much more extensive ink.
Bizarre Sexual Dimorphism: The women look like extremely pale-skinned humans, while the males look more like their Zabrak ancestors note though Zabraks are classified as a "near-human" species with horns on their bald heads and darker/more colorful skin pigmentation.
In contrast, the canon established that the few guerrilleras we saw under Mother Talzin's command in The Clone Wars were actually about all the (wo)manpower the Nightsisters had before being nigh-exterminated. Indeed, the Nightsisters were scary enough that the local Imperial Warlord blew up every working ship on the planet to keep them from escaping into the larger galaxy and wreaking havoc. A single high-rank witch like Silri lent the Zann Consortium many Nightsisters who mastered Force drain and rode rancors, forming a shock troop contingent so powerful that it could go toe to toe with Imperial land forces, and other posterior clans were still powerful enough to be threats to Jedi and Sith alike, including Luke Skywalker of all people.
Adaptational Wimp: In Legends, even after being decimated by the Empire, the Dathomirian Nightsisters were capable to mobilize impressive forces with minimal coalitions.