I might be reciting what some people ahave already said but here goes:
It's been all-but-confirmed that Solas, Fen'Harel, The Dread Wolf, whatever you want to call him locked away the Elven Pantheon. Cole sees this is him, he says to Solas when reading his mood:
"He hurts, an old pain from before, when everything sang the same
you’re real and it means everyone could be real
it changes everything but it can’t
they sleep, masked in the mirror, hiding, hurting, and to wake them… (Gasp!)"
Solas also makes mention of an "Old Elven skirmish," and "A mistake" that he made when he was young. Solas locked away the Old Elven Gods. I agree with Knight-Enchanter's theory. Fen'Harel was a 'God' of rebellion. He rebelled against the Pantheon, being neither one of the 'Creators' or 'The Forgotten Ones". He didn't like what the 'Gods' were doing to Elvhenan. Factions warred among themselves, Gods went mad and hurt people, and slaves were kept in service to different deities.
So, he lead a rebellion against them, and locked them away. Again going off what Knight-Enchanter says, I think the Creators are locked within an Eluvian, in an alternate world. One that is not ours and also not the Fade. Cole says they are "Masked in the Mirror," so this seems to make a lot of sense.
So, I think Solas lead his rebellion and, thinking himself successful, slept. He underwent Uthenera. And when he woke, he was horrified. Humans had taken over the world, elves lost their homes, their history, their beautiful magic, and their immortality. Those that remained lived in squalor or as aloof, savage Dalish, who had so long misread and misinterpreted Elven history that they were barely still "elves' by Solas' definition. Solas had tried to improve his people's lives by ridding them of their Gods, but instead, he brought his people to ruin. A certain way of playing through the Temple of Mythal reveals that Tevinter did not destroy Arlathan, elves did, with centuries of brutal infighting. Without their Gods, they were lost. So, Solas is devastated by what he now knows was a mistake, that mistake being the locking away of the Elven Gods. It is this mistake he seeks to rectify. His Orb is the Key to releasing the Gods he locked away so long ago. It's the Key to the Eluvian, or something quite like an Eluvian, that holds back the Creators. He allows Corypheus to acquire it to unlock it's power, and joins the Inquisition in hopes of defeating Cory and regaining his powered Orb.
We know the rest.
Two things, to me at least, are left outstanding.
1. What happened to Mythal?
Her and Solas are clearly aquatinted and not at all antagonistic towards one another. Were they allies in the days of Arlathan? And what of her "Murder?" She was not locked away like the rest of the Creators. I think her murder was at the hands of the other member of the Pantheon, and it was this act that spurred Fen'Harel into his great Rebellion.
2. What happened to the "Forgotten Ones?"
We now know that the Creators were hardly the most benevolent Gods, so the Forgotten Ones must have been pretty awful. But they too were locked away. Here's what I think happened though. I don't think that Fen'Harel locked the Creators and The Forgotten Ones in the same place. Elven legends, however misinterpreted, say that the Creators were locked in "The Heavens" while the Forgotten Ones were sealed in "The Abyss." To me, "Abyss" says underground. What other Gods are underground? The Old Gods, that's who. I think the Old Gods of Tevinter are The Forgotten ones in Dragon form, or at least aspects of the Forgotten Ones in Dragon form.
This explains why they would drove the Magisters of Tevinter to breach Heaven and usurp what they found there. Stay with me. So, I'm also becoming a bigger and bigger believer in the concept that the Black City is, in fact, Arlathan. NO traces of Arlathan have been found, and legends say the city "Sunk" when Elvhenan fell.
I think that Arlathan is where the Creators are imprisoned in the fade. This is why no one but (supposedly) the Magisters of Old have been able to reach it. The Black City IS real, it is always present in the Fade, but none can reach it. Except, I think, by access ranted by Solas' Orb.
This would explain the Ancient Elves connection to the Calling. And I do think that is exactly what the message of the Well refers to. I have no doubt that "The Calling" the voices speak of does refer to THE Calling.
The Calling of the Forgotten Ones, the Old Gods, calling out from their prisons in "The Abyss" to crack open the prison of The Creators and continue their ancient battle.
I think Cory was part right, part wrong, and part insane. I think that that when Corypheus and the Other Magister breached "The Golden City" they DID find nothing but chaos and dead whispers. I think that after so long in a blacked Aralthan, that's all the place would be. But I do think they were cast out. Not by the Maker, but by the Creators imprisoned there. As a blow against the world that they had once ruled, a world that turned against them, and locked them in this eternal prison in the "Heavens."
One last thing. I believe that the act of sealing away the ancient Elven deities created what we know as the Veil. Again going off the Theory of Knight-Enchanter; Solas speaks so fondly and so readily of a world where the spiritual and the physical are one, one might be forgiven for thinking he himself has lived in such a world. Furthermore, what Cole says about Solas lends itself to this idea, "...an old pain from before, when everything sang the same. You’re real and it means everyone could be real..." I believe that in the days of ancient Arlathan, there was no Fade, no veil.
TL;DR:
-Well Message does refer to THE Calling.
-Fen'Heral did lock the ancient Elven Gods away.
-Creators were locked inside of Aralthan and banished to the Fade, "The Heavens."
-Arlathan is the Black City.
-The Forgotten ones were banished underground, "The Abyss."
-Old Gods of Tevinter are, or are strongly related to, the Forgotten Ones.
-Old Gods/Forgotten Ones "Called" to the ancient Magisters to enter the "Golden City of Aralathan and usurp what the found there; "The Creators."
-Sealing away the deities created the Veil and the Fade.