The Nightsisters are a coven of witches that lived on the mysterious world of Dathomir. The sisters ruled unchallenged, gifted with the power to wield dark magicks. The men of Dathomir, the Nightbrothers, lived apart from the witches, but were subservient to their whims. Despite their power, the Nightsisters avoided the conflict of the Republic and Sith, content to pursue their own matters. The Jedi gave them a wide berth, for they remain focused on matters on Dathomir. Count Dooku has had dealings with them in the past, and turned to them when he was in need of a new apprentice. Their betrayal of Dooku earned the Nightsisters a powerful enemy, and he ordered a massive attack by his droid forces in retribution.
Mother Talzin was the spiritual guide of Dathomir's Nightsisters during the Clone Wars. She sold the services of the Nightsisters to the galaxy's wealthy and powerful, and wasn't above backstabbing any clients who treated her clan with disrespect.
Asajj Ventress was born to the Nightsisters of Dathomir. As an infant, she was handed over to a slaver who, in turn, died during a pirate uprising on Rattatak. When Asajj was later abandoned by Count Dooku, she returned to the Nightsisters, seeking their help.
After Count Dooku betrayed her, Asajj Ventress went to the Nightsisters for aid in her quest for vengeance against him. Mother Talzin helped Ventress, earning Dooku's vengeance, which in turn led to an intense battle that wiped out the clan.
Savage Opress was once a common Nightbrother who was transformed into a monstrous warrior by Mother Talzin. The Nightsister leader gave Savage a new quest: to find his brother, Darth Maul, who had somehow survived his apparent death and was living in exile in the Outer Rim.
Old Daka was the oldest and wisest of the Nightsisters. She alone could wield their most powerful magicks, with the ability to even animate the dead.
General Grievous led the devastating onslaught of Dathomir. His battle droid forces overwhelmed the Nightsisters, wiping them out, while he himself slew their wisest member.
Old Daka's powers stirred the dead of the Nightsisters' graveyard, creating an army of the dead that swarmed the battle droid forces.
The world Dathomir is naturally abundant with dark magicks, and was the source of the Nightsisters' power.