Thedas is obsessed with singing. "lyrium" itself, as was pointed out on reddit, is potentially derived from Latin words that we owe things like "lyrical" to as well. this is all very much hooked into the Maker: the Andrasteans do the whole 'Chant of Light' and are specifically obsessed with song as a means of worship, even if they can't write lyrics for shit. SOME DAY SOON THE DAWN WILL COME.
it's implied that this Maker-song force is, if not actually malign, at least not what it appears to be. at one point in Inquisition the Templars--of the non-EBIL variety--are described as being hooked into something old and hungry through their lyrium addiction, which is what gives them their (incredibly shitty) antimage powers.
in Thedas dwarves can't do magic ever because they have no connection to the Fade, but that also makes them, along with Tranquil mages who've been taken out of the Fade's address book, uniquely capable of handling lyrium. lyrium is way poisonous in its unprocessed form, but dwarves and Tranquil are immune to its effect (largely--there's a dwarf in DAO who got lyrium poisoned, survived, but sort of seems senile now, not sure if that's been retconned or what, because in theory it shouldn't be possible). Dagna's a character from DAO who shows up again in Inquisition, a dwarf who loved magic and went off to study it even though she couldn't do it herself. if you quiz her as you hand things off to her throughout the game, she actually has an insight into the nature of lyrium and becomes "bigger than herself" for a second, like she has a momentary connection to the Fade. she speculates that since only dwarves and Tranquil can work with lyrium, they must be connected in some way, and further wonders if lyrium isn't like a river--if it can touch you, it wants to drag you along with it, which in this case means 'kill the shit out of you.'
during Flemeth's cryptic speech toward the end, she says something along the lines of "so long as the song keeps playing we must all dance to the tune." this is shortly before it's revealed that she is what she is because she shares the soul-ish thing of an old elven god and steals the archdemon soul (I hate that word and I hate having to use it repeatedly in a short span even more) from Morrigan's WORST FUCKING VOICE ACTOR AND DIALOGUE IN TIME kid. I imagine this doesn't necessarily happen if the kid doesn't have the soul in your game--haven't tried it that way, never will, fuck Inquisition. relevant in a couple paragraphs will be a line here: Morrigan has been shrieking about how Flemeth wants to steal her body, and just before leaving Flemeth says (good delivery and facial animation, bad dialogue) that "no body can be forced to accept a soul, you were never in any danger from me."
what does definitely happen in every game is the epilogue, where Solas, your "I lub the Fade and spirits" mage, goes to visit Flemeth in some elven temple-looking otherspace. she calls him by the name of the old elven god who is (falsely, according to an immortal elf you run across) thought to have imprisoned all the other elven gods outside the world. given that this is just the trailer for their ongoing blurt of glacial storytelling, we've actually seen this implied before (there was a statue of Solas' godsoul thing in the temple of Flemeth's godsoul-thing; if you find it, educated NPC companions can point out 'hey, wasn't he supposed to be a traitor?' and faithful NPCs go 'ANDRASTEANS PUT ANDRASTE'S BETRAYER IN THEIR TEMPLES' and players are meant to go HMMM I WONDER IF WE'VE BEEN LIED TO.
Solas reveals that he'd given Corypheus "the orb" that Cory used to prolapse the sky--it'd broken after what we might laughably call the climactic encounter and Solas had been suspiciously miserable about it. he tells Flemeth he couldn't unlock its power after "his long slumber." he says he deserves to die for what he did, but that "the People" (capitalized, elves, presumably) need him. he and Flemeth get all close and exchange "I'm sorry"s, and then something ambiguous happens: there's a soul-exchange lookin thing like when Flemeth snatched the OGB's meta-essence (which appeared not to harm OGB's human side at all). the wiki thinks that this is Solas stealing Flemeth's god-nature, but in light of the "only a willing body can accept a soul" line, I think this is actually Flemeth possessing Solas, who is clearly "willing" in that he thinks he deserves to be punished with extermination. Flemeth's original body dies; Solas' body, ambiguous who's running it, lowers it to the ground as it appears to turn to stone, for who knows what reason.
so what the fuck does it all mean? my guess, to take the most wildly speculative approach, is that "the Creator" was actually some possibly-eldritch entity who showed up on the scene late in Thedas' prehistory and didn't like how things were run, or possibly an as-yet unspecified ancient elven deity who got ideas above its station. it kicked the (other) elven gods' asses but couldn't quite kill them, so they ended up semimythical aspects of the world, mostly dormat. when the Tevintir entered the Golden/Black City, the Maker didn't cast them out; it actually fucking exploded, which is when lyrium entered the world (this would be a good place for citation--is there solid evidence lyrium has always existed? not sure there can be, actually, since histories in DA are often deliberately written to be wrong). some semblance of the Maker still exists, or maybe lyrium can just act in concert--but what it really wants is to converge on itself again, which process seems to involve, either deliberately or as a side-effect, killing the shit out of people who work with lyrium. it's also possible that, rather than wanting to be restored, it wants to die and take the world with it, rather than existing in its fractured state. on some level, it's capable of manipulating and orchestrating events at a very large scale. this is "the music."
meanwhile, the gods it betrayed (Flemeth refers to her godsoul-buddy as having been betrayed repeatedly, but not by the Solassoul-buddy legend thinks did it) have started waking up. Flemeth's has been around for a long time; Solas' appears to've come to just recently. realizing the scale of the threat that the shattered Maker poses, Flemeth and her godsoul buddy devise a plan: they need to perform a convergence of their own, collecting up into one body enough of the ancient power of Thedas (another theme of DAI, Morrigan is obsessed with how much more magical the world used to be) to opposed and dissolve the Maker before it can either recompose itself or pull off the end of the world. by the end of DAI, Solas' body contains at least two and possibly three ancient godbuddies--at least the two elven ones and, if you had the OGB, one of the Tevintir old gods.