The Clone Wars S03E12: Nightsisters

Troubled by Asajj Ventress' growing prowess with the dark side of the Force, Darth Sidious commands Count Dooku to eliminate her. But Ventress survives Dooku's assassination attempt, and the jilted former apprentice vows to take revenge, enlisting the aid of her kinswomen -- the mystical Nightsisters -- in her sinister scheme.