Mother Talzin was the leader of the Dathomirian Nightsisters and the current "mother" to Asajj Ventress, who took Ventress in after Dooku betrayed her.
Mother Talzin, the spiritual guide of Dathomir's Nightsisters during the Clone Wars, holds enormous power on her homeworld and wields the strange magics of the dark side.
When Asajj Ventress was betrayed and broken, Talzin took Ventress in and when Asajj told her the whole story, she immediately had a plan. Talzin got two other Nightsisters, Naa'leth and Karis, to assist Asajj in an assassination attempt. Using magic to make the trio invisible and arming them with Jedilightsabers and a special poisonous dart, Talzin sent them to attack Dooku at his estate on Serenno. When the attempt failed Mother Talzin used the attack as oppurtunity to offer Dooku a new "apprentice" manipulating the Sith Lord's new found paranoia brought about by his nearly being killed in his own home. When Ventress found a candidate, Savage Opress, he would become Dooku's new "apprentice", but his real job was to eliminate Dooku for Talzin and Ventress.