Mother Talzin, the spiritual guide of Dathomir's Nightsisters during the Clone Wars, held enormous power on her homeworld and wielded the strange magicks of the Nightsisters. She sold the services of the Nightsisters to the galaxy's wealthy and powerful, but wasn't above backstabbing any clients who treated her clan with disrespect. She earned the ire of Count Dooku by aiding one of her sisters, Asajj Ventress, exact revenge against the Sith Lord.
Mother Talzin commanded the Nightsisters, and was the most powerful of their order. The coven looked to her for advice, and it is through her that all interactions with the galaxy at large occurred.
Mother Talzin welcomed Asajj Ventress back into the order after Ventress' betrayal by Count Dooku. Talzin actively wanted to cultivate Ventress' hatred and plans for vengeance.
Talzin and Dooku have had past dealings with each other -- enough so that Dooku felt confident he could ask for her assistance in procuring a new apprentice. When Talzin helped Ventress seek revenge against Dooku, she earned a powerful enemy.
Talzin led the Nightsisters in their transformation of Savage Opress from a lowly Nightbrother into a hulking, dark side warrior.
Talzin tasked Savage Opress with a quest and equipped him with an enchanted amulet that would lead Opress to his goal: his long lost brother, Darth Maul.
Though Talzin led the Nightsisters, she was not the wisest of the coven. In times of great need, Talzin turned to Old Daka, the eldest of the Nightsisters and keeper of the most arcane, dangerous knowledge of the clan.
The world of Dathomir, a planet steeped in dark energies, is home to the Nightsisters. It is here that they were most powerful, and they rarely ventured off-world in the age of the Republic.