Crassius Servis is encountered during the The Trouble with Darkspawn quest.
After exiting Coracavus the Inquisitor can confront Crassius Servis at the nearby Echoback Fort where Servis will attempt to counter-attack with several Venatori. After he and his comrades have been defeated, he can be spared which will give the Inquisitor the opportunity to judge his fate.
Servis willingly discloses his operations as a Venatori but admits he has little real loyalty to the organization, being little more than a third-party hireling paid to smuggle ancient Tevinter artifacts, and offers himself to the Inquisition instead.
- See also: Skyhold prisoner dialogue