I read the Last Flight, and I don't remember anything being mentioned wrt to lyrium and dwarven Templars, even though they appear (and dwarven wardens) . Also have a feeling that while that book is great for fleshing out larger lore issues, I wouldn't cling to small lore details in it since it was an outsourced book. (Absolutely worth reading, I loved it )
Oh god, a dwarven Mage villain...now that would be interesting! (And give dwarves that much desired relevancy for the main plot, lol)
TEWR, I'm too tired to parse out your theory that you're too tired to expand on, lmao, but I look forward to reading over it tomorrow/seeing any further thoughts you have. I've been drafting my big dwarven lore/lyrium/stone meta, so I'm def in the headspace (overall not right now lmao) to think about it!
I suppose while I'm up right now I can give it a crack. I've got some energy, though I'm not much of a meta person. I dabble but I'm not all that great lol, partially because the DA wiki ends up lagging horribly and also I have an atrocious train of thought. However I'm sure when you have the energy you can pick up my trail. I'll also post it on tumblr and link you into it
So we know lyrium is alive. This has been very much the belief of mine and the Dwarves since DAO. The Dwarves believe the Stone guides them, its will protecting them and granting them the means to survive. Nowhere does this become more apparent then the Crosscut Drifters codex, who were searching for a large amount of ore but were letting "the Stone guide them". And guide them it did, bringing them to the flank of a large amount of Darkspawn that would've threatened Orzammar's safety had their tunneling succeeded.
Beyond that there's also the Gangue Shade codex, talking about the "waste and unstable rock" which "pollutes the stone".
The Stone has a will that surrounds and directs; she guides even when we are willfully blind to her influence. But she is not pure. The Stone bears a corruption as old as balance. For the dwarves to prosper, the gangue--the waste and unstable rock-- must be cut away. But like the Stone, the gangue also has an influence. Each of us must face this, must carve the worst of ourselves away, but the Legion of the Dead bears a unique responsibility. Only the fully adorned of the Legion can face the gangue, can cut into darkness that afflicts the raw Stone. She encircles us, and we must protect her, here where darkness meets light.
--A Legion of the Dead inscription, undated
It has implications of meaning Red Lyrium to some people, I suppose. I think what's more fascinating to me is the idea that the Dwarves have for a while known about the Primeval Thaigs and covered it up. The Codex on the PT says it was from a "sealed archive in Orzammar" and I would find it hard to believe that a scavenger would've traveled from Orzammar to Kirkwall and back again. The very codex mentions that the idea of the Memories being wrong is very disconcerting, so I would find it very believable that they'd cover it up.
We also have Caridin's Journal, which mentions that he was granted a "vision by the Ancestors". I remember an OLD Gaider post talking about how the Dwarves can dream, but do not do so in the Fade. However I'm hard-pressed to find such a thing. Oghren and the DW dream, though that could just be due to the Taint as they're both Wardens at the time.
At any rate, Caridin using such a line can't be mere coincidence. The Anvil, as we see, isn't something that could just be created by a Dwarf. It's wound into the lyrium, and the Dwarves have tales of Golems -- ancient statues animated by the souls of the dead.
Interestingly, where else do we see Golems? Why in the PT of course! Indeed, the presence of Golems there indicates that they were either brought there by an earlier expedition many years ago (Bartrand did have information on the thaig) or that they have always been there. I believe the latter.
The PT is old. I believe it was stated somewhere that it's around 10,000 years old at least. If I had a PC version of DAII I'd take a screenshot of the statues therein (I want to say they might appear similar to Mythal statues in DAI), but regardless the presence of Golems indicates that the PT society had used them before... and indicates that the Anvil of the Void might also come from such a time. Factoring into this is my belief that a PT exists somewhere near Orzammar.
The Veil is also very thin in the PT, evidenced by the presence of Shades. The Red Lyrium Idol can thin the Veil, per Anders, and is especially horrible at doing so if it's fractured. With Red Lyrium widning all through the thaig and growing from the Stone, I'd hazard a guess that all of it is contributing to the thin Veil -- to say nothing of whatever calamity befell the Dwarves before.
After all, said Dwarves ended up "feasting on the magick stones" -- per the Hunger Rock Wraith Abomination (which is a title reserved only for possessed mages) -- that they considered their gods. We know little of what befell the ancient Dwarves and can only conjecture. Perhaps they engaged in some cultural taboo and were sealed away, or perhaps they breached the Golden City somehow and lost their magic as a result.... and those Dwarves who weren't involved cast the ones who were away for their treachery and these Dwarves desperately wanted to regain their magic.
Or perhaps it's the other way around. Those who betrayed the Dwarves left the ones who hadn't to wallow in the PT. Perhaps the story Shaper Czibor tells us (at this point, it's reasonable to take any origin story as possibly being true) of the founding of the Dwarves has something to do with it, though he says the Dwarven belief in the Stone dates back only 2000 years (which wouldn't be accurate considering the Elves and Dwarves knew of each other and IIRC 1000 years before the Divine Age Tevinter was in power).
We who are forgotten, remember,
We clawed at rock until our fingers bled,
We cried out for justice, but were unheard.
Our children wept in hunger,
And so we feasted upon the gods.
Here we wait, in aeons of silence.
We few, we profane.
—Found scrawled on a wall in the lost Revann Thaig by explorer Faruma Helmi, 5:10 Exalted. Unknown author.
Profane means outside that which is considered sacred, so they had... what... done something that cast them out of the good graces of the Dwarves of their time? They did something that was considered irreverent and disrespectful? More I think on it, the more I become convinced they're related to the Golden City turning black and decadent with corruption (as Corypheus saw).
Yet the Rock Wraiths are more then simple creatures. The one encountered by the Legion of the Dead took out the Darkspawn and then collapsed the entrance to the tunnel where it dwelt, doing nothing to harm the Legion of the Dead.
I also want to talk about the Darkspawn, who ended up defending the entrance to the Primeval Thaig with a ferocity that was... strange. Nathaniel Howe and his Wardens had been assured the way would be clear, but the Darkspawn around there
were anything but. That the Thaig is untouched by the Darkspawn... and that they would defend it but not enter it... well that says volumes. They don't care about other places yet they don't enter the PT proper.... merely defend the outer areas.
Indeed, Shaper Czibor recounts how the first Darkspawn seen were like them in appearance (Genlocks), though this contradicts with Caridin's journal where he believes the Chantry version -- which is funny considering the Chantry version hadn't really popped up yet. Personally, given the evidence that suggests the Darkspawn did come from beneath the Dwarves and how Red Lyrium is blighted, I consider Czibor's to have more weight to it.
I'm not saying the Memories can't be wrong, as there's precedent that the Dwarves have covered up the truth (Cadash Thaig's history, Bruntin Vollney's desires, etc.), only that we have a lot of info to support the Dwarves being responsible. Indeed, it's not unreasonable to assume the Shaperate had some notes similar in nature to Caridin's journal's talk of the Darkspawn, considering Amgarrak was the result of people taking what few notes they had from Caridin's life and trying to make more Golems.
Side note, if that does happen to be the case I can't say i'll be surprised or disappointed. Since it had been my theory from the get-go it won't be a big deal to me.
Anyway, another theory I had was that each race was responsible in some way for the emergence of each type of Darkspawn, but I abandoned that one a long time ago. Although who's to say the Darkspawn didn't come from somewhere else? Or that they're a problem on other continents? The Dwarven Empire extended all the way towards Par Vollen after all. Perhaps it extended further then that as well.
Another thing I had forgotten is that the red lyrium idol used in the final fight (when it's a sword) gives off the same whispering sound the Darkspawn use, the Joining uses, and the Reaver Joining uses.

I assume there's a connection, as to reuse a sound effect associated with the Darkspawn in all of these things would be rather lazy to me.
Back to lyrium (I apologize for the mess this post is bound to be) and its song, it sings not only to the Dwarves but also to Justice. indeed, we hear a faint hum when we give him the ring. Red Lyrium also sings "a darker song" according to Cole, and according to the Red Templars it sings "a deeper song". Even Bartrand and Varric heard the song (Meredith simply heard a voice, though I don't believe it was said she heard a song. If she did she may have assumed it was the Maker) and if those two could hear it perhaps Dwarves in general can hear it. We know lyrium, regardless of color, sings to them. Cole talks of how
Since it's Blighted, it's connected to the Song of the Old Gods that draws the Darkspawn. FWIW, Andraste is also noted to have a connection to songs (which is why I believe she's a Dumat OGB Somniari Blood Mage.... whoo that's a mouthful).
We also have to account for House Valdasine, somehow. An entire house kept the Dwarven empire supplied with lyrium on their own -- which would require a lot of trade expeditions, finances, resources, etc. -- and yet one day they shut their doors and never opened them. When they finally did open, they were gone. All gone, and only a magic staff like lyrium and metal that chilled the heart and filled it with sorrow remained.
What if the disappearance of House Valdasine is somehow connected to the emergence of the Darkspawn? What if they discovered red lyrium and were tainted by it?
Also supporting the "Dwarves were once mages" idea is that Genlocks have mages within their ranks, who manipulate the Taint in their system to cast magic. The Taint acts as a mana alternative and obviously it has power. Avernus realizes this and so does the Architect -- with the former discovering some "alarming implications" about it all.
As it stands, I'm a lore expert with theories but I'm unable to really pinpoint the threads connecting them to each other in a proper way. Perhaps that's due to how the lore doesn't really intersect that much for these theories or perhaps it's due to my mind being a mess.
All I know is my gut says maybe.
Wait, no, I'm not President Neutral.
All I know is that from what we have to go off of, the PT seems to be connected to the emergence of the Darkspawn. Certainly the Blight.
EDIT: Oh yeah, and Morrigan comments on how the corruption in the Circle Tower is akin to that of the corruption the Darkspawn produce.
EDIT 2: Also Malvernis, whose corruption is like that of the Darkspawn only it's more virulent -- and possibly he predates the Darkspawn. We're never given a timeframe on when he was imprisoned. But he also takes the form of a dragon -- an ability reserved only for those who held a special place in the Elven religion.
EDIT 3: Oh yeah, Cole dialogue.
The temples have a lot of keys. They all sing, but never the same song -- for the Storm Coast
Cole: I like traveling with Dwarves.
Varric: Glad to hear it, Kid.
Cole: You're quiet, but the old song still echoes inside, almost like Templars.
We know from Gaider that Dwarves are barred from the Fade, but not immune to being in it entirely. Cut off, so it's said, and indeed Cole says as much I believe -- though I know not where. Dwarves can become Templars and even Spirit Warriors -- which are not dissimilar schools really -- and are granted abilities magical in nature.
Solas finds the fall of the Dwarven lands confusing, though he talks little about it, which makes me wonder what brews in that egg of his.
Dagna's commentary on how she felt as tall as a mountain is interesting. I take this to mean she was feeling the Stone. She says she was moving, and the Dwarves say the Stone guides them and the Ancestors return to it and make it stronger. She thought all the thoughts. She was tied into the grand memory of the Dwarves, all that binds them when they die, the voice of the ancestors. Sigrun did mention that the Dwarves say the Memories of the Stone are forever... and lyrium is used to create the memories by the Shaperate (something she remembered in fact).
She caught a glimpse of what the Dwarves used to be -- mages. Beyond that....
"Lyrium and the fade -- linked. But Dwarves and Tranquil, not linked. But they work lyrium so they are. Somehow."
"Like the lyrium needs to flow, but if you're part of it, it takes you with it. So you can't be part of it"
This seems to say to me that the Tranquil, whom we know are branded with lyrium, but are taking the Dwarves into them (tying into my lyrium = Dwarven souls theory above or something) and thus they're neutralizing everything. Or something.
Dagna said it best though.
Words are mush.