Asajj Ventress is a Force-sensitive assassin trained by Count Dooku. She was created for the Clone Wars multimedia project and first appeared in Dark Horse's Star Wars: Jedi — Mace Windu comic (2003), then in Genndy Tartakovsky's Star Wars: Clone Wars micro-series later the same year, and was voiced across the 2008 series by Nika Futterman.
Two origins, and they do not reconcile
This is the sharpest continuity split of any character on this site. In Star Wars Legends she is Rattataki, from the war-ruined world Rattatak, apprenticed to a stranded Jedi named Ky Narec and turned to the dark side when he was killed. In current canon she is Dathomirian — a Nightsister, taken from her clan as an infant and sold, then raised on Rattatak, and eventually returned to the sisterhood that made her. Same assassin, same master, entirely different person underneath.
The discard
What both versions share is the central injury: Dooku throws her away. Ordered by Sidious to abandon her, he does it without hesitation, and everything she becomes afterwards — bounty hunter, Nightsister, someone else's problem — is shaped by having been disposable to the one person who claimed her.
Dark Disciple
Christie Golden's Dark Disciple (Del Rey, 2015) is her essential novel, adapted from unproduced Clone Wars scripts. It pairs her with the Jedi Quinlan Vos on a mission to assassinate Dooku and turns into the strangest romance in canon Star Wars: two people, both compromised, each convinced they are the one saving the other.
Reading note
Read the canon version and the Legends version as separate characters who happen to share a face. Trying to merge them produces nonsense, and the Nightsister material only works if Dathomir was there from the start.
