According to The Real Barenziah, Book II, Barenziah had slept with him to gain entry to Thieves' Guild. He not only told her about the skills she needed to become a member of the Thieves Guild, but also trained her himself, as well as introducing her to other members. Among them was a woman called Katisha.
One night, Therris summoned Barenziah and Straw to loot an estate, which turned out to be General Symmachus' estate. Straw was set outside as a lookout while Therris and Barenziah sneaked in. However, when two figures entered the room and caught the thieves, Therris betrayed Barenziah and ran, but was caught shortly afterwords. The captor turned out to be Symmachus himself.
As a punishment for betrayal, Barenziah suggested twenty stripes; she did not want to do him any permanent injury. However, Symmachus had Therris drawn and quartered. His severed head was later mounted on a spear and exposed on the walls of Riften.