"Right. In Haven, Corypheus basically bragged about being one of the magisters that breached the Golden City. Not that there's any way to confirm this, but with the power he has, breaking open the Fade, it's possible. At any rate, the corruption is obvious. He'd been kept in some Grey Warden prison forever, until breaking out. It's in one of Varric's books, since he was there when it happened."