Asajj Ventress is the minor antagonist from the Star Wars Expanded Universe. She is voiced by both Grey DeLisle in Star Wars: Clone Wars and video game and Nika Futterman in Star Wars: The Clone Wars.
Assaj Ventress used to be Dooku's assassin until he, and his master Darth Sidious betrayed and got rid of her because they were tired of Ventress's failures during every Clone Wars battle. But really, Sidious demanded her death, fearing she was becoming too powerful in the Dark Side and that Dooku might be training her as a Sith Apprentice, and would betray him.
Ventress survived and returned to the Nightsisters' clan she'd been born too long ago and was welcomed back. However, several attempts of revenge on Dooku led to the slaughter of the clan and Ventress was forced to flee and eventually became a bounty hunter.