I’d like to add something, if you don’t mind!
The “old dreams” are probably the Titans. Everyone assumes that Solas did what he did because of the Evanuris, that they were his major concern, and that is true only to a certain extent.
The Evanuris were certainly dangerous and Solas even had personal reasons (revenge for Mythal’s death) to finally end them once and for all, but the biggest issue was that the world was in danger. The Evanuris were destroying it, little by little, and Solas had to intervene before it was too late and Thedas couldn’t be saved anymore.
The elves realized that the Titans’ blood, lyrium, had special enhancing magical properties, something that the Evanuris would have undoubtedly desired to have for themselves to strengthen their rule over the People. We know that their war against the dwarves lasted long and that it was bitter and bloody: the Titans attacked by shaking the land and destroying the elves’ cities.
Elgar'nan, Wrath and Thunder,
Give us glory.
Give us victory, over the Earth that shakes our cities.
Strike the usurpers with your lightning.
Burn the ground under your gaze.
Bring Winged Death against those who throw down our work.
Elgar'nan, help us tame the land.
──Song to Elgar'nan, found in the Temple of Mythal, author unknown
The elves directly struck the Titans down and mined their bodies (Elgar’nan apparently was the first with Mythal who won over the Titans and elven slaves were even forced to build a giant statue to celebrate his victory within a day). The elves apparently also enslaved the dwarves/Tal-Brytol’s people - the workers of the Pillars of the Earth - as well.
Now, it’s not hard to imagine what the elves found and why they found it. The Blight might be a self-defense mechanism used by the Titans to counterattack and defend themselves and the dwarves. A last resort to destroy the elves, which also resulted in the Titans’ own poisoning.
The Evanuris didn’t want to stop and, as @eluveon and @rederiswrites said, they even started to utilize the Blight, not caring about the consequences (just look at Andruil). That’s when Mythal and Fen’Harel intervened, sealing the dwarves underground, hiding the lyrium excavation center we find in Trespasser (filled with their statues positioned as if they are guarding something).
Mythal is probably the one who severed the dwarves’ bond with the Titans and erased any mention of them in their history, causing the future generations of dwarves to forget entirely about the protective beings living deep below them and to believe Elgar’nan’s Sun was ready to scorch them whenever they went up on the surface (and thus leading them to believe that that surface = wrong and dangerous; underground = only place where the dwarves can live).
Ir sa tel'nal,
Mythal las ma theneras.
Ir san'a emma.
Him solas evanuris.
Da'durgen'lin,
Banal malas elgara.
Bellanaris, bellanaris.
I am empty, filled with nothing(?),
Mythal gives you dreams.
It fills you, within you(?),
Making our leaders proud.
My little stones,
Never yours the sun.
Forever, forever.
Solas created the Veil not just to lock away the Evanuris and probably the Blight itself (which then Corypheus and the other magisters unleashed back on the world. This would also explain why the Blight seems to have originated both from the Fade and the Deep Roads.). He created the Veil also to let the Titans stay dormient.
It’s the Breach, a sudden and giant tear in the Veil, that causes the Titan in The Descent to wake up abruptly. If/when the Veil falls down, the remaining Titans (the majority of which are Tainted by now) will awaken fully. They are already aware of how much things have changed and are going to change: the Breach allowed them to awaken, to “find the dreams again”, and now they patiently await for the final, definitive hole in the sky which will let them have their revenge upon the elves.
The “old dreams” might be the Titans.
The “Old Gods” dragons might be a measure of security, a bait, put there by Mythal to keep the Blight at bay. Basically, she sealed the dwarves underground with the same plague their protectors had created and made sure none of it could come to the surface, forcing the first generation of darkspawn to look for the dragons, in an endless, maddening search. This is a huge, big “?” theory, though, but it would explain why Solas is so against the idea of killing all the Old Gods. He knows they are the only thing keeping the darkspawn down there.
Red lyrium (Tainted Titans’ blood) is a direct consequence of the Blight and it probably came together with it.
If this is exact, then things are shaping up to be even more complicated and dangerous than we thought. We will see several factions involved in DA4: Qunari versus Fen’Harel, ex-Inquisition versus Qunari, ex-Inquisition versus Fen’Harel (with more or less strain depending on the relationship with him), Qunari versus ex-Inquisition and Titans versus Fen’Harel and probably the whole world.
It’s not reassuring to know that the Titans are as large as a continent and can rely on magic too. So my personal reaction to this is: