The Black City is a Titan
I'll be pouring a collection of thoughts on the topic, then I'll tie them together in a short explanation.
In elven lore, we learn the Evanuris, in their greed, unlocked something they shouldn't. This something awoke and was full of wrath, and its power was said to be able to destroy the world. It was then sealed and forgotten for aeons.
In DA2, we find the Primeval Thaig, from where red lyrium seems to sprout. The idol was there as well, and it seems to be the key that opens the door, path or gateway - eluvian??? - that leads to that place. We found no eluvian in the Primeval Thaig, but it must have been there somewhere, and that eluvian must've been the doorway into the Black City.
Now, whatever that power in elven lore was, it's probably the source of the Blight, which was unlocked by the 7 Magisters who entered the Golden City. Why do I say this? Because Corypheus said he found only chaos and corruption within the City. The darkness, as he calls it, was already there.
We also know for a fact red lyrium is lyrium infected with the taint. And lyrium is the blood of a Titan. We also know Titans are not humanoid creatures. They are literally cities hidden in the earth's womb, accessible only by those who know the path to their location.
In Descent, after we leave Shaper Valta, the path we took to find the Titan is forever lost. What if the same happened to other Titans? What if they were only accessible through eluvians, and only those who had the right keys could open them, such as high priests?
Now let's gather our thoughts.
We are told the Black City is a place in the Fade containing only dead whispers and the taint. It is probably only accessible through a specific eluvian located somewhere in the Primeval Thaig and the key to unlock it is the red lyrium idol.
The reason why the Black City is located in the Fade must be related to the fact that part of Arlathan existed solely in the Fade, such as the Vir Dirthara (the library we access in Trespasser). When the empire collapsed, so did the once Golden City and the pathways that led to it.
In the afrescos we see in Trespasser, there are seven "mirrors" or eluvians connected to a city. Those could be the original seven eluvians used by the ancient elves to directly access the "Golden City" (or whatever they called it in their lore). That city or Titan must have been important, since, as stated repeatedly throughout the franchise, the Black City is visible from all locations in the Fade. THIS MEANS THE BLACK CITY IS AT VERY HEART OF THEDAS. It must be a focal point from which the world was born and formed. And if it is a Titan, then that Titan is the source of life in Thedas - and possibly of magic as well.
Somehow, the very life source, the wellspring from which all life is formed became corrupt. And if it is so, then it means the very core of Thedas is corrupted, which would justify Solas' agenda of wanting to destroy the world and rebuild it anew.
Why is that Titan a wellspring? Simple. Remember the Well of Sorrows? It's a magical wellspring connected by several eluvians, each leading to a different destination; possibly to different temples (located within Titans) and/or capitals of the ancient elven empire. I believe the Titan which became the Black City served as the original paradigm for the ancient elves to build their temples of worship to each elven god.
Also, notice how water is connected to memory in the lore - priests like Abelas drink from the Well and can access the memory of their ancestors - and how drinking lyrium, which is liquid, connects dwarves to Titans and old memories, possibly from their ancestors and from a time before Arlathan fell.
As for the seven old dragon gods aka Archdemons locked in prisons deep beneath the earth, they could be guardians protecting the seven eluvians that lead to the Black City. That would make sense since the darkspawn are attracted to the dead whispers, just as anyone who is infected with the taint, meaning the Evanuris locked within the Black City - or somewhere in between, only accessible through the Black City - are also infected with the taint and communicate through it, placing their victims under a powerful spell which instructs them to dig beneath the earth and find the eluvian to unlock it and finally release them.
I have a feeling DA2 was supposed to be a step forward in understanding the lore behind the Black City, the taint, eluvians, Archdemons, etc. Instead, the game was rushed and we never saw that idea come to light. There was even a dragon protecting the Primeval Thaig and the idol. Maybe it was an old dragon, still not infected?
As for Inquisition, it was supposed to bring answers to all these issues by probably tying everything to ancient elven lore - I mean, the Inquisitor is literally counselled by an old elven god. I guess we'll have to wait to see if DA4 finally brings some sense of closure to this madness.
I just rewatched a gameplay that shows the unknown ruin, where we learn how Mythal stroke down the pillars of the earth and freed the People (presumably elves) from the domain of what we can infer were the Titans.
There's a description of vines covering up a sphere of fire and things being calm for aeons.
But later, this thing awakens and wrath irradiates from it. Which takes us back to the beginning of the post.
Now, we met something full of wrath which then later becomes calmed down in Descent, didn't we? It was the Guardian, the stone sentinel that kept anyone from reaching the heart or core of the Titan aka the pulsating thingy at the city's centre.
You'll also notice how the design of the place where the Titan's Heart/Core lies is VERY similar to the one from the Well of Sorrows. Hence my parallel between Arlathan's places of worship and Titans.
How does the Titan calm down? By killing its Guardian and allowing it to connect to "one of its Children". So a mental connection to a lifeform calms the Titan down.
However, as far as we know, dwarves didn't exist in the time of Arlathan. Then how did Mythal calm down the Titan she faced after presumably killing its Guardian?
She must have "connected" to it in the same way.
It's entirely possible she allowed the Titan to share a mental connection with her if it's what it took to save her people. And that act could've been represented by the vines covering the sphere of fire.
So when Mythal was murdered, the Titan she was connected to sensed her death. And it probably caused it to wake up and feel wrathful at her murderers.
Being connected to someone for aeons creates a powerful bond. Maybe it even caused Mythal and that Titan to merge. They slowly became one entity without realizing it. So when Mythal died, to the Titan, it likely felt as a personal insult. As if the Evanuris had killed a piece of them. An important one. And it filled them with rage.
The bond with the Titan might also play a major role in her immortality. Remember, Solas did say nothing in elven lore connected his people to the dragons. So maybe the secret of effective immortality kept by the Evanuris referred to being bonded not to dragons, but...to Titans?
About the Black City/Titan's wrath, remember how red lyrium is described by Cole (I think) as being angry? The song it emanates probably reflects the Titan's anger at Mythal's murder. Many millenia later, It still echoes throughout the ages.
The big question everyone is asking: where are the Evanuris locked? If they were in the Black City, why didn't Corypheus find them? Why weren't they freed right there and then?
They're not locked in the Fade. They're locked someplace IN BETWEEN.
Remember Morrigan showing the Inquisitor the place she stayed in after leaving the Warden and entering the eluvian? That eluvian led her to a place neither in the Fade, nor the physical world, but rather SOMEWHERE IN BETWEEN. Somewhere perhaps like The Crossroads, where we finally walk upon in Trespasser.
That is why Corypheus heard their whispers, but never found anyone. The Evanuris were locked away behind the eluvians leading to the Black City, their voices reaching him from their prison. And because he didn't have they key to open the eluvians, he couldn't meet them in person.
(Imagine the magister's shock if he learned his gods were actually elves, beings considered to belong to the slave cast in Tevinter)
The Evanuris are likely locked away someplace within the Crossroads, from where they can only leave by opening the right eluvian(s) with a key. And that key is now in Solas' possession: the red idol.
Those are my thoughts, anyway.