I'm a little bit late for the magister discussion, but I'd like to add a correction to Kissiebear's post about the magisters hearing whispers.
According to the Canticle of Silence (pages 58-60 in WoT vol. 2), the whispers of the Old Gods actually stopped being heard by the faithful AFTER the magisters breached the Golden City. After being scolded and cast out by the Maker, even.
Specifically in the Canticle, it was the one acolyte who decided not to follow through with the ritual who still heard the whisperings of the Old Gods, up until he got to Minrathous and the magisters were being kicked out of the Fade.
This is why it would make sense that those who "whispered" manipulated the Magisters by appealing to their pride and arrogance. Those that claimed to be the Old Gods were actually imprisoned in the Golden City, which is why Corypheus said "I found only chaos and corruption, dead whispers, for a thousand years I was confused, no more." I'm inclined to believe that he unleashed "The Forgotten Ones" who are actually the ones who were whispering. Once they were freed, they had no need to whisper.
Here's a quote from Geldauran, one of "the Forgotten Ones" talking about the Elvhen Pantheon
"Their pride will consume them, and I, forgotten, will claim power of my own, apart from them until I strike in mastery."
I'd also contend that they needed the Magisters to come into The Fade physically not only to breach the Golden City, but also in order to have hosts (physical vehicles) with which to cross the veil back into Thedas (the physical world) much like parasites. I believe the blight stems from them and the reason darkspawn search for the "Old Gods" (Great Dragons) is because they (The Old Gods) are in fact The Elvhen Pantheon's bodies or physical representations of them. They are said to be sleeping because they are in a type of forced Uthenera and their spirits are locked away in another dimension (probably somewhere in The Fade, but not necessarily) only accessible through the eluvians. Given what we know of the antagonistic relationship between the Elvhen Pantheon and "The Forgotten Ones", we could understand the motivations behind such actions as well as the irony found within them. I also believe the veil was created to secure and imprison the Elvhen Pantheons' "essence" while keeping them separated from their bodies. I believe this is why Solas is so concerned and appalled when the Grey Wardens mention slaying all of the remaining Old Gods to prevent the blights.
Conversations between Solas and Cole seem to allude to this
Cole: They sleep, masked in a mirror, hiding, hurting and to wake them... (Gasp) where did it go?
Solas: I apologize, Cole. That is not a pain you can heal.
(Cole refers to the Elvhen gods, masking or masked from their true aspects in an eluvian, sleeping and suffering. When he says where did it go, he refers to Solas' pain which Solas in turn confirms by stating that Cole cannot heal it. This is quite interesting. It would seem that the Elvhen god's might not be imprisoned in the Golden city, as I suspect, but rather in a realm reached only through an eluvian).
Cole: "He hurts, an old pain from before when everything sang the same" (before the veil?)
Cole: "You're real, and it means everyone could be real. It changes everything. But it can't."
(I think this is a reference to him being physically present after the veil was created, as in bonded with Solas like FleMythal. With this knowledge, he could help his brethren do the same, however, if he does, then things will become what they were and Fen'Harel can't allow that, despite the love he holds for his kin. This potentially adds weight to my theory about the spirit/physical separation of the Pantheon)
So when an Archdemon is awakened, it's actually a Forgotten One who takes control by usurping an Elvhen god's body, corrupting it from within.