Decimus is a character featured in Dragon Age II.
Not much is known about Decimus's past. He used to be a mage at the Circle in Starkhaven before it burned down. Some other mages in his group suspect that he is the one who caused the fire. He also seems to have an affair with Grace.
Decimus is involved in the quest Act of Mercy. Ser Thrask will send Hawke a letter asking him to come to a remote cave along The Wounded Coast. When Hawke arrives, Thrask will tell him that a group of apostate mages who escaped from Starkhaven are hiding inside. Hawke must get them to surrender or else Ser Karras will arrive and 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 a man named Decimus. Decimus has taken up blood magic and is ready to fight to the death for their 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 and so are some of his mages. However, a few mages (Grace 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.