Corvinius is one of the praetorians from Vitellius family and thus commanded by either Vibius Vitellius Lurco or Tiberius Vitellius Scaevola. He was a Greek pirate before being hired by Vibius. He is basically the rogue of Vibius and performs stealth missions for him.
He is first encountered during the quest Divide and Conquer in the first act, when he has contact with Mithridates VI Eupator. You will also find him in the Troas fishing village for the storyline quest The Smuggler. He has to be beaten in order to advance the quest. He will turn himself invisible, but he be revealed when an ally stands next to him.
Egypt[]
In the second campaign, he will be encountered againg during Diplomatic Distractions.
During the siege of Alexandria, he will be encountered again in the final battle, in which he assists you and his patron Vibius Vitellius Lurco and afterwards, he assassinates the bookkeeper who can act as witness for embezzlement and fraud.
Gaul[]
In Gaul, he will be encountered during the quest In This Mess, where he, Marcellus and Vitellius Lurco and the player group fight against a suprise attack in the village.
The second time he will be encountered is during the quest Ambiorix's Warning, in which the player can control Vitellius Lurco group and must defeat the remaining Gauls.