The elven gods weren’t truly gods. Maybe they were powerful mages or powerful nobles. But they had god-like status among their people, which is enough (imagine inverted commas around the word god in the rest of this). And of course they were immortal like the rest of the ancient elves.
The society of the ancient elves isn’t what the Dalish think it was. They idealize, romanticize and cling to it because so much has been lost to time. From TME we know there was classism and the keeping of slaves iirc?
The elven gods ruled over their subjects/worshippers/slaves in a not-great way, even the so-called good “Creators”. Nobles marked the faces of their slaves to show which god they followed. Andruil had periods where she descended into madness. Elgar’nan was possessed of an all-consuming rage. Ghilan’nain destroyed the creatures she had created with her power - all but the halla pretty much - in order that the gods would ascend her to godhood. There’s something about Dirthamen and dodgy blood magic too iirc. At times the gods warred among themselves - Andruil and Mythal have fought for example. I can imagine this warring extending to their followers. And at some point Mythal was murdered. So not an ideal society at all (this isn’t even getting started on the Forgotten Ones, who may have been even worse). And this is why Fen’Harel rebelled.
Fen’Harel is a god of rebellion, not trickery. The ancient root words of “harellan” mean “struggle” and “noble opposition”. Alone out of all the gods, Fen’Harel did not approve of how they ruled over the People. He wanted the People to be free from tyranny. He thought he had a good idea of how to solve the problem - and indeed to him it must have felt like the only way. He tricked both groups of gods and sealed them away, made them fall asleep, in a realm beyond, reached via Eluvian. All but Mythal, who either was already dead (only to surface later of course, as we know the spirit of Mythal came to join with a human woman) or who had already come back as Flemythal and wanted vengeance and so joined Fen’Harel in his plotting. Solas both in romance dialogue and in dialogue with non-elf inquisitors that he disapproves of makes reference to how he knows the mistake of trusting so well that he could carve her face, how it’s been so long that he had a [non-spirit] friend that he could trust, etc. Either one of the other gods betrayed him, or he felt betrayed by the way they ruled over the rest of the elves.
Morrigan tells us Eluvians can be opened and locked and each one has a key. In elven history, orbs called foci were used to channel the power of the gods. They were usually associated with a member of the pantheon, and required a great deal of energy to unlock. Fen’Harel used his foci to lock the rest of the gods away in an Eluvian. That orb is the key to re-opening that Eluvian. After doing so, he went to sleep, uthenera. He must have been very tired from expending the energy needed to use the orb. He may also have been very sad. He did what he did for the greater good of the People, but he couldn’t escape the way doing what he did must have felt. He had just banished all the rest of his kind from the world and was now alone. Dying alone is his greatest fear. Wolves are pack animals, after all. The pain and sadness was such that he preferred to fall into a slumber than exist alone.
Unfortunately, his plan backfired. Abelas tells us that it wasn’t the Imperium that destroyed the culture of the ancient elves - the ancient elves destroyed themselves. Without their Creators they descended into war and destroyed themselves. Without their Creators they were further destroyed by humans. Long after this happened, Fen’Harel awoke. In horror he realized what he had done, what his actions had caused - elves are now enslaved in Tevinter, subjugated in alienages, persecuted and killed by humans, or wandering the wilds as but shadows of their former glory, clinging to scraps of the past and being seriously mistaken about ancient elven lore. He wanted to free them, but thanks to what he did, they’ve virtually lost everything. Their history, their power, their culture. He regrets sorely what he did and references this in dialogue - a mistake made by a younger elf. He tried to make things better by sharing his knowledge with modern elves (the Dalish) but as we hear from him in dialogue they turned him away, called him mad, wrong, etc. It makes him think that the modern elves aren’t his people, the ancient ones are. All of this is a huge part of why Solas is filled with such quiet sadness and loneliness, and a huge part of what Cole senses when he touches Solas’ mind and sees the pain there. A pain that he [Cole] can’t heal, Solas tells him.
Fen’Harel wanted to right his wrong, fix his mistake. He tried to use his orb to unlock the Eluvian and let the elven gods back into the world, but he was too weak when he awoke. Desperate, he gave the orb to Corypheus, since Corypheus had enough power to activate it. I suspect he gave it to Cory because he was the only being he was aware of with enough power to do so. Cory is an ancient being too after all. This as we know inadvertently caused the explosion.
It’s kind of ironic because Cory too awoke from slumber and found the world gone awry.
Solas joins the Inquisition to fix this second great mistake. And fix it they do - they seal the Breach and defeat Corypheus. But the orb lies broken and Solas is distraught, because it was the key to releasing the others.
He returns to Mythal. He should pay the price for his mistakes, and/or the “price” of what will be needed now to open the Eluvian - what that is I cannot say. But the People - the elves - he thinks they need him. His intentions now are to find another way to carry out his plans of unleashing the gods again.
I’m not clear on what happens in the final scene. Either Flemythal is sorry because of what she’s about to do him - possess his body - (though I think this is unlikely since he speaks to Lavellan through Cole after this occurs) or he’s sorry for failing and for what HE’S about to do which is kill Flemythal and take her power. This power will help him in his quest to unleash the gods and Flemythal in this scenario isn’t truly dead because 1) she’s Flemythal, powerful, a DA staple and 2) she probably has another Horcrux thing somewhere. OR they’re both just sorry at everything that’s happened and the poor state of the elves, and Flemythal willingly gives him her power.
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…
He’s in so much pain, an ancient pain. He wants to right what he thinks was a wrong and put things back the way they were. Alone among all the elves of modern Thedas, Lavellan is different, Lavellan listens to him share his knowledge, Lavellan is unique and world-changing and a magnificent spirit, she’s too bright, like counting birds against the sun, the mark makes her more and yet it wasn’t what made her this way - maybe he’s misjudged the Dalish. She changes everything - if there can still be elves like her, maybe the gods don’t need to return to the world. But it can’t. He has to do it. He has to fix his mistake and bring them back.
solas, bright and sad, observes and accepts. spirit self, seeing the soul, solas, but somehow sorrows
(variation on the above: Solas isn't Fen'Harel exactly but an elf who lived centuries ago (like he said, from a village to the north where he grew up as a young man, and like the ruin Leliana's spies find remnants of) and the spirit of Fen'Harel persisted through the ages after what it did to the gods and eventually came to join with Solas as Mythal did with Flemeth. Solas would have met him in his wanderings of the Fade.
Or you know he could just be Fen'Harel. either or)