In the world of Dragon Age, Corypheus is one of the originators of The Blight and darkspawn. He was one of the infamous Tevinter Magister Lords who travelled to the Golden City to overthrow The Maker, corrupting it to the Black City. He was cast back down to Thedas (seemingly retaining his sense of self unlike regular darkspawn).
Sealed away for centuries by the Grey Wardens, Hawke and companions must confront him after he is set free in Dragon Age II DLC, Legacy.
Dragon Age II- Legacy
After being sealed away in a dormant state by Malcolm Hawke in the Grey Warden prison tower, Corypheus' magic began to seep out and corrupt everything around it, which is partially implied to be responsible for the number of blood mages and criminals in Kirkwall not far away. One of the groups possessed, the Dwarven Carta, set out to obtain the one thing that could free their "master"- the Blood of the Hawke. With Malcolm Hakwe having been long dead by the time, the Carta set out to obtain the next best thing, and kill Hawke and their siblings Bethany or Carver. Their attack fails, and prompts Hawke to get the squad together and seek out the Carta.
When they arrive and learn of Corypheus, they venture through the tower prison, slowly uncovering the story of Malcolm Hawke's sealing away of the demonic monster and the Grey Warden's surprise at the sentience it showed. During the ascension of the tower, Hawke meet with the Grey Warden, Janeka, who believed she could harness Corypheus as a weapon and control him, and the former Grey Warden commander, now reduced to a simple ghoul with unusual vocalizing skills, Larius. Unlike Janeka, Larius wants to slay the "sentient Darkspawn".
Regardless of the side Hawke chooses, the Champion of Kirkwall is forced into a fight with Corypheus. Moments before, it is made apparent that Corypheus is not a Darkspawn, as they had been lead to believe, but one of the Tevinter Magisters that went to the Golden City and blackened it. However, unlike the Chantry's version of the tale, Corypheus claims that he and the others ventured at the call of Dumat, one of the old gods and the first Archdemon in the history of Thedas, and that he was horrified to discover the Golden City was blackened when he arrived. The trauma seemed to have destroyed his mind, at least partially, and when he was suddenly awoken from his slumber he flew into a panic that he could no longer hear Dumat's voice. Hawke was able to defeat the Magister, but the event left all of them shaken, espicially Anders, who began doubting his entire world-view as a result of his encounter with Corypheus. It is also heavily implied that Corypheus was not slain, and leapt to the body of either Janeka or Larius after Hawke defeated his initally form.
While these events never became common knowledge, the Chantry Seeker Cassandra calimed to have troops at the Vimmark Mountains that could not learn of the Champion's activities there, claiming that Varric Thethras was not telling her the entire tale of Hawke to her. Varric brushed this off, claiming that it was just family business, and that he didn't think it would matter, but decided to share the events with her anyway.