Taliesen is a member of the Antivan Crows, a renowned guild of assassins operating out of Antiva. He survived a Tevinter shipwreck off of Llomerryn as a boy and was bought by House Arainai and inducted into the Crows in 9:12 Dragon. He and Zevran were two of the eighteen recruits purchased that year, and the only two to survive.
If Zevran's life was spared and he was recruited by the Warden, then the party is ambushed by Taliesen and his men in a Back Alley in the city of Denerim, after you arrive with Arl Eamon to attend the Landsmeet. Taliesen reveals that he has come to look for Zevran, having volunteered to track the rogue assassin down on the Crows' behalf. If Zevran is not in the current party at the time of the encounter then he will appear to address his former comrade, or not at all if his life was not spared.
Taliesen offers Zevran the chance to return to the Crows, prepared to concoct a story to cover for the elf's long absence. However, to do this they would have to finish Arainai's original mission and kill the Warden. At this point Zevran may turn on the Warden if his approval is not high enough (+33 or at least "Interested" in a romantic involvement), and he will be permanently killed in the following confrontation.
If Zevran's disposition towards the Warden is high, he will honor his oath and decline Taliesen's proposal. He says farewell to Taliesen, declaring he is saddened it has come to this. Depending on the Warden's dialogue options, Zevran may either stay to help in the fight or decline to draw arms against his old companion (if he was in the active party, he will leave until the fight is over). After all is said and done Zevran concludes that the Crows will assume he rejoined Taliesen and subsequently died with him, thus giving him a chance to disappear before they learn otherwise. Despite this he will ask to remain with the Warden to see the Blight through to the end. Accepting his request yields six approval points.
Zevran has romantic feelings for Taliesen, which becomes apparent whether or not he betrays the Warden. Indeed, Taliesen, Zevran and Rinna were romantically involved, their relationship having developed while working together as a unit within the Crows. If Zevran accepts his friend's offer, he mentions that he's glad Taliesen came for him (as opposed to some other Crow he didn't know, presumably) as it "makes things easier"; if he refuses, he expresses sadness for Taliesen's death, admitting that he was a good friend, "and more".Should the Warden take the time to get to know Zevran, he may reveal that Taliesen was a key player in the events that led to his leaving Antiva. During the course of Zevran's final jobs with the Crows, Taliesen implicated their third accomplice - a woman named Rinna who Zevran had become infatuated with - of accepting a bribe from their target. Taliesen killed Rinna with Zevran's approval, but afterwards it was discovered that Rinna had not been conspiring against them. Zevran left for Ferelden soon after on his mission to kill the Warden, eager to find his end. Taliesen was sent to Ferelden to find out what happened to Zevran on orders from Grandmaster Eoman of House Arainai.