Decimus used to be a mage at the Circle in Starkhaven before it burned down. During that time, he had a relationship with Grace. Decimus felt like a prisoner in the Circle, and Grace credited him with giving her and the other apostates the courage to fight the templars. Some other mages in his group suspect that he is the one who caused the fire at Starkhaven.
This section contains spoilers for: Dragon Age II.
Decimus is involved in the quest Act of Mercy. Ser Thrask sends Hawke a letter asking them to come to a remote cavealong the Wounded Coast. When Hawke arrives, Thrask tells Hawke that a group of apostates who escaped from Starkhaven are hiding inside. Hawke must get them to surrender or else Ser Karras will slaughter them when he arrives. After entering the cave and eliminating the early threats, Hawke finds the apostates at the back of the cave. The group is led by Decimus. Decimus has taken up blood magic and is ready to fight to the death for his freedom. He assumes Hawke to be a templar and attacks. Hawke kills Decimus and the few mages that fight by his side.