Not much is known about Decimus's past. He 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.
Decimus is involved in the quest Act of Mercy. Ser Thrask will send Hawke a letter asking him to come to a remote cavealong the Wounded Coast. When Hawke arrives, Thrask will tell 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. After entering the cave and eliminating the early threats, Hawke will find 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 quickly presumes Hawke to be a templar and attacks. Hawke kills Decimus and the few mages that fight by his side.
Decimus is killed along with some of his mages. However, a few mages (Grace, Terrie and Alain) survive by not taking arms against Hawke. These mages can either be convinced to go back to the Circle, or aided in escaping. If the latter option is taken then most are recaptured and sent to The Gallows. Alain will later state that some were turned Tranquil.