Barriss Offee is the main antagonist of the last four episodes (and therefore the final antagonist) of the fifth season of the TV show Star Wars: The Clone Wars.
She is a Mirialan Jedi Knight, General and Healer. She carries a blue lightsaber (later Asajj's two red lightsabers) and was a friend to Ahsoka Tano, as well as a mentionable character in Season One and a secondary character/supporting protagonist in Seasons Two, Three and Four before becoming one of the main antagonists in Season Five.
Barriss was once the loyal Jedi Padawan of Luminara Unduli and skilled healer. In the period of the Clone Wars, she earned the rank of Jedi Knight and General. However, as she was fighting in this galactic conflict, she slowly realized how bad it was. She believed (quite rightly) that the Jedi became corrupt and had always been the true ones responsible for the Clone Wars; that the Jedi are the real villains, the real ones to be put on trial. This led her to secretly turn to the dark side.
Fueled by delusion and immoral, to show her protest, she launched a secret bombing on the Jedi Temple. She forced Letta Turmond to feed explosive nano-droids to her husband, Jackar Bowmani, who worked in the Jedi Temple as a munitions expert. Next day in the hanger, nano-droids blew up, killing Jackar and six Jedi as well as clone troopers. All evidence led to Letta and she is sent to the prison. When Ahsoka came to see her, Letta was Force-choked to death by Offee, but it was Ahsoka who ended up in the prison for "killing" her.
Offee later entered the prison, killed several guards and helped Ahsoka to escape, with her believing it was her master, Anakin Skywalker, who helped her. Unfortunately, Ahsoka was blamed for killing the guards and had to flee and attempt to prove her innocence. She contacted Offee, who pretended to help her and led Ahsoka to a warehouse. She then knocked off Asajj Ventress, stole her mask and lightsabers and attacked the Togruta, disguised as Ventress. Their duel ended when Ahsoka fell off a balcony, landed near crates of nano-droids and was arrested by the clones.
Shortly afterward, Ahsoka was brought before the Jedi Council, who proceeded to expel her from the Jedi Order, stripping her of her rank, title and battlefield privileges, and turned her over to the Republic military courts so she could receive more "impartial' judgment.
Meanwhile, Anakin, who correctly believed Ahsoka was innocent, hunted down proof for the real culprit. He questioned Ventress who told him that Ahsoka was in contact with Offee. He came to the Mirialan's quarters where she attacked him with Ventress' lightsabers. He eventually managed to capture her.
Anakin brought Offee to Ahsoka's trial, saving his Padawan from capital punishment. Offee told the truth to the court. She fiercely stated she felt that the Jedi were corrupt, arrogant and were the ones responsible for the Clone Wars. Afterward, she was imprisoned while all charges against Ahsoka were dropped. The young Togruta was personally invited by the Jedi Council to rejoin the Order, but she refused and left, having lost faith in the Council for not having such in her.
It's unknown what happened to Barriss Offee after her trial (in the canon version of Star Wars), but it's possible that like all other Jedi, she was seen as a threat to the coming Empire (even though she was not a member of the Order any longer) and was killed along with other Jedi during Order 66. Her former master, Luminara Unduli was betrayed and killed. Her bones were later used by The Grand Inquisitor to lure the Jedi who survived Order 66 from their exile.
Barriss Offee is the only Dark Jedi as the final fate is unknown (all others are dead).