Ventress was found on the savage planet of Rattatak by a marooned Jedi. He trained her in the force, her parents having been killed, but was forgotten by the Jedi and eventually died. Ventress developed a huge vendetta against the jedi and began tapping into the dark side of the force. When count dooku came to a gladiator match on Rattatak, he took Assajj as his new commander and pupil, charging her with the assignment of eliminating anakin skywalker
Spoiler:. She later reappeared attempting to frame mace windu as an assassin. her next moves have been the implimentation of a biological weapons test on one of Naboo's wounds and she was later seriously injured while protect the vaccine to the agent with Durge. Her fate remains to be seen.
She then lured anakin away from the battle of munilist but was defeated by him in a duel on Yavin IV shortly before the revelation of General Grievous. this is from the clone wars cartoon