During the Clone Wars era, the Nightsisters embarked on a mission with the objective of eliminating Darth Tyranus, the Dark Lord of the Sith, known publicly as Count Dooku, after he betrayed his apprentice, Asajj Ventress. Ventress, accompanied by Karis and Naa'leth, journeyed to Castle Serenno on Serenno, driven by a desire for revenge, but the Sith Lord successfully defended himself, overpowering his attackers.
During the Battle of Sullust, Darth Sidious communicated with his Sith apprentice Darth Tyranus, expressing his growing concerns regarding the power wielded by Dooku's assassin, Asajj Ventress. To appease his master and avoid admitting he was training her to eventually overthrow him, Dooku commanded his forces to abandon Ventress to her apparent death. A salvage crew discovered her in her damaged Ginivex-class fanblade starfighter, and she seized control of their ship, returning to her homeworld of Dathomir. There, Mother Talzin consented to assist in exacting revenge upon Dooku, providing poisons and potions that granted invisibility.
Now cloaked in invisibility, Asajj Ventress and her two Nightsisters landed in the forests surrounding Dooku's castle on Serenno, infiltrating the fortress through an air vent. After locating their target, Ventress fired a poison dart that struck Dooku's neck, causing him to lose his sight, although he could still sense them through the Force, mistaking them for Jedi. Ventress initiated an attack on Dooku using a lightsaber, commencing their duel within the count's bedroom, which eventually spilled into his office. Despite maintaining her disguise throughout the confrontation, Ventress managed to disarm the Sith Lord and was poised to deliver the final blow, but the count defended himself by unleashing Force lightning upon Ventress and her companions, propelling them out of a window and over a cliff, thus ending their assault. Nevertheless, the three Nightsisters survived and made their way back to Dathomir.
Dooku, due to the Jedi lightsabers used by his attackers, wrongly assumed that the Jedi had attempted to assassinate him. In an effort to protect himself, he began searching for a new apprentice to take Ventress's place (believing, mistakenly, that she was dead).
Following the failure of their direct attempt to assassinate Dooku, the Nightsisters devised a fresh strategy to eliminate the Sith Lord by utilizing a double agent. To put this plan into motion, Mother Talzin reached out to Dooku, offering to provide him with one of the Nightbrothers to serve as his new assassin. Dooku, seeing this as a chance to acquire the new apprentice he desired, accepted the offer. After selecting Savage Opress to be their pawn, Talzin presented the imposing Zabrak to the count, who was pleased with Oppress, unaware that he was secretly loyal to them. These loyalties were hidden even from Opress himself, but were later to be activated by Ventress to aid her in another attempt to kill Dooku.