The Star Wars saga is immense in its lore and backstory, which should come as no surprise given that material has been continuously published ever since the first film released in 1977. The Nightsisters are a perfect example of a group with a rich, fully developed history that is superficially touched upon in Star Wars Jedi: Fallen Order.
The story of the Nightsisters begins 600 years before the Battle of Yavin (BBY), which is when Star Wars: Episode IV - A New Hope takes place. A member of the Jedi Order was exiled to the planet of Dathomir, a place that 1000 years BBY had an active Sith Temple, and the Dark Side of the force was strong in the surrounding area.
Over time, the women of the planet began to harness the force, but in a way that was distinct from both Jedi and Sith, considering the abilities conferred as magic and spells. Those who used spells that were too rooted in the Dark Side were banished, and over time, these group of banished individuals became known as the Nightsisters. They cultivated their own beliefs and cultures for centuries, never straying far from Dathomir as their powers weakened if too far away.
The Nightsisters connection to Fallen Order is a tragic one, as during the Clone Wars the entire clan was massacred by General Grievous, sent by Count Dooku. Few escaped for a time, including the clan leader Mother Talxin, the assassin Asajj Ventress, and a few others.
We first learn of the Nightsisters within Fallen Order when Cal lands on Dathomir, and one of the few survivors, Nightsister Merrin, works to keep him away. Merrin blames the Jedi for the loss of her clan, but later sees that Cal refuses to turn to the Dark Side of the force and aids him in defeating an enemy. Now aware that she was being manipulated, Merrin joins the crew of the Stinger Mantis to help Cal in his journey.
It will be interesting to see if Merrin appears again in future Star Wars content, because she is both one of the last of her clan, but also one of the most powerful. Merrin is one of the most capable manipulators of Nightsister magic. Her skills allow for her to enhance the power of her fellow clan mates in combat, teleportation, and most important of all, she has the power to resurrect dead Nightsisters, though the people brought back resembled undead husks animated by the force and nothing of their former personalities.
With Merrin no longer bound by the manipulation of others on Dathomir, she is free to do as she pleases, and one route she may take is to rebuild the Nightsister clan. If her character is not further developed, it was still great to see the Nightsister assist Cal in his journey throughout Fallen Order.