Corypheus is a darkspawn magister. The Inquisitor reported that he could jump into the bodies of other darkspawn and those who have the taint. So while you did kill him, you also didn't. I'm not sure he tricked the Venatori mages other than making promises he probably wasn't going to deliver. He promised them they'd be god-kings, ruling over all. Yes, that Fiona. She was lied to about a Templar army bearing down on Redcliffe, where the mages had taken refuge. Instead of confirming it with her mages or asking Arl Teagan to send out scouts, which I'm sure he would have done had she asked, she leaped into the arms of a Tevinter magister. And I can only hope she didn't know what the Venatori did to the Tranquil under her care.
And before you ask, there was no Templar army bearing down on Redcliffe. There were renegades causing trouble out in the Hinterlands, along with a fair number of renegade mages. But the Templars had all been recalled to Val Royeaux and were then taken to Therinfal Redoubt where an envy demon was planning on exposing them all to red lyrium.