Quinlan Vos was a Republic's agent undercover, who had infiltrated the inner circle of Count Dooku's Dark Acolytes, pretending to have defected from the Jedi Order and fallen to the Dark side. His initial assignment was to be a spy for the Republic, but after some time he began to see his main purpose in discovering and possibly eliminating the mysterious "second Sith," Dooku's teacher, whom the Count had mentioned to Obi-Wan Kenobi on Geonosis. Gradually Vos was becoming obsessed with this idea, of which Count Dooku, unbeknownst to Vos, was quite aware.
When Dooku started a conversation with him about Sith traditions, Vos made a try to find out the second Sith's identity, mentioning that Dooku couldn't become a Sith Master while his own master was alive. Though Dooku wouldn't reveal anything, in the course of this talk he gave Vos a mission to go to Coruscant and to kill someone; with obscure hints he made Vos believe that the target was in fact the second Sith, whom Dooku was now ready to get rid of. Vos was all too eager to fulfill the mission, believing that he would serve the Republic with that.
Quinlan Vos arrived to Coruscant disguised as a Kiffar general Zac'ryah Vos, accompanied by Khaleen Hentz, also disguised as a Kiffar. Upon his arrival he received from Dooku the identity of his target, and that was Senator Viento.
In the dead of night Vos got into Viento's apartment; hiding his presence in the Force, he successfully managed to pass not only Senate Guards, but also Master K'Kruhk, who was guarding the Senator as well. Nevertheless, as soon as Vos ignited his lightsaber, K'Kruhk heard it and smelled ozone, and Viento began to wake up. Anxious, that somebody might stop him, and expecting Viento to put up a ferocious fight, Vos hurried to finish his mission as soon as possible.
Quinlan didn't understand his mistake until it was too late; only after delivering a killing blow did he realize that he wouldn't be able to eliminate Viento so easily if he had really been a Sith. At that very moment K'Kruhk charged into the room and attacked Vos, accusing him of betraying everything he had been taught. They jumped out of the window onto the roof of a hover train, where Vos wounded K'Kruhk badly, pushed him off the train and escaped. After that, he returned to Dooku, wanting for some answers and telling him that Viento hadn't been Sith at all, at which Dooku seemed amused and stated that he had never said that, and that Viento had only been his agent who had tried to double-cross him.
Though it was discovered after Viento's death that he had been in league with the Separatists and had been involved into sabotaging Loyalist senators' speeders, the very fact of assassination almost convinced the Jedi Council that Vos had really fallen to the Dark side.
Viento's body was frozen in carbonite and placed in the Senatorial Tombs. A year later, Quinlan Vos, having returned by that time to the Jedi Order, entered the Senatorial Tombs to use psychometry on Viento's corpse in order to find information about Finis Valorum's death, in which Viento had been also involved.
- The New Essential Chronology