During the First Contact War in 2157, General Desolas is tasked with a secret assignment on Shanxi: a turian research vessel has crash-landed on Shanxi with an important artifact aboard, and Desolas must recover it. Desolas leads a squad into an abandoned human settlement on Shanxi while searching for the crashed ship, where he is ambushed by the human mercenaries Jack Harper, Ben Hislop, and Eva Coré. Desolas' squad is killed and Desolas himself is taken prisoner.
The mercs use a datapad Desolas had with him to locate a turian camp. Harper uses a fake message from Desolas to lure the turians into a trap. When the mercs find the cave where the artifact that the turians were looking for has been hidden, Harper and Hislop investigate while Eva guards their turian prisoners. After fending off guardians, Hislop touches the artifact and is struck by a bolt of energy and Harper is blinded. Outside the cave, Eva is distracted by the strange lights, allowing the turians to overpower her. Back in charge, Desolas takes Eva and Harper prisoner, and orders his soldiers to take Hislop's body away for study.
When the First Contact War is brought to an end by the Citadel Council's intervention, Desolas returns Eva and Harper to General Williams on Arcturus Station. He expresses interest in Harper due to his exposure to the artifact, the Arca Monolith, but Williams prevents him from detaining Harper any longer.
Desolas is encountered again on Illium, after Harper uses his connection to the Arca Monolith to pursue it there. Desolas reveals he was aware of Harper's connection, and used the Monolith as bait to recapture Harper. Taking Harper and Eva to Palaven, the turian homeworld, Desolas explains that he plans to use the Arca Monolith to convert the turian race into an army of super-soldiers that he can use to dominate the galaxy from Temple Palaven, a revered turian site which conceals another Monolith. Harper argues that Desolas doesn't understand the Monolith's true purpose, but the general ignores him and sets his plan into motion.
Desolas is stopped after Harper convinces his brother, Saren Arterius, that Desolas is endangering the turians. When Desolas' converted turians begin turning against him, he becomes trapped within Temple Palaven while Harper makes his escape. Saren contacts Palaven Command and orders an orbital strike to destroy the Temple and prevent a "bioweapon emergency". Temple Palaven is destroyed, and Desolas and the Monoliths with it.