@vbibbi: I won't pretend to know - but there are indications in the codexes that imply a concept like this.
- we know the evanuris hunted and killed Titans (and dwarves, their minions).
- we know they mined lyrium
- we know lyrium is the 'blood' or 'veins' of a Titan
- we know red lyrium is tainted by the Blight
- we have accounts of Andruil hunting "Forgotten Ones" which we know to be "wicked things that thrive in the abyss" - we know she put on an armor of the void, drifted into madness and her lands became plagued. I believe it is not a far stretch at all to say that she hunted darkspawn - contracted the blight - as did her lands - and was cured of it by Mythal.
- we know that the Evanuris encountered foul things that scared them so deeply (think on that - it scared these beings) that they collapsed the mines leading to the lyrium and never spoke of it again.
I believe that as revenge, or through some encounter with the Evanuris, a Titan or something akin to them (my thought is that the Titan's are tied to Thedas - and Thedas made the Blight - but that is PURE conjecture) became blighted... and created the darkspawn (which were first just blighted dwarves/genlocks?)
Morrigan also observes two things that might be relevant:
Flemeth is somehow connected to the Blight. Which, if the story of Andruil and the armor holds any water would make sense.
And that the taint in the Circle Tower during Broken Circle looks much like darkspawn taint.
We also know that Seekers are immune to possession due to the Vigil, but it also seems they're resistant to red lyrium.
There's also the fact that demons are spirits warped from their purpose. What if that's what the Blight is, or a part of it anyway?
One other bit: the Claw of Dumat in the Fade at Adamant looks exactly like architecture found in two dwarven Thaigs. Bartrand's Folly, and Heidrun Thaig. Both predate the First Blight IIRC.
As for factions, saying Darkspawn are the most powerful faction in Thedas is somewhat like saying the Black Plague was the most powerful faction in Europe at the time. With few exceptions, darkspawn don't think. They need a guiding intelligence. Other than that, they're feral animals, very very nasty ferals, but feral none the less.
I'd say the Evanuris if only impact. Truth be told, we don't know all that much about the Old Gods and what they really are.
Good money says that's what we find out next. Interesting that Dumat taught humans a way to use magic that would not depend on Lyrium. Just one last thought.