I cannot believe that I hadn't even considered Coryfish-as-Larius until you said that. I've been trying to figure out why Solas took one look at the blight corrupted magister and went 'yup. Seems trustworthy.'
And this is after
Ok ok, I'm catching up to the thread and saw this comment.
After dealing with Cory, he possess either Janeka or Larius, whoever left alive.
Bianca contacts a Grey Warden mage after she finds out red lyrium is blighted lyrium -> that Grey Warden is the one you met in legacy and left alive. Varric deduced that whoever she met was possessed by Cory after he's been 'killed' by Hawke.
Sooo... This raises a lot of questions.
When did this happen, on timeline?
How long did Cory walk around looking like a (relatively) normal human?
If Solas met Cory while he was wearing another person's skin, why would he give him the orb (he would meet a simple mage)?
Solas wouldn't have given the orb to just some power hungry mage, so how did Cory convice him? (Though not looking like a hideous monster would help Solas make said stupid decision)
What do you think Cory did with the red lyrium?
Cory's appearance changed a bit between DA2 & DAI -> the lyrium shards that were sticking out of his body turned red.
So... Did he ingest the red lyrium?
Was he infected by touch? That'll be weird, Cory already had the blight after all 
I personally think that Cory's appearance was heavily altered by high dosage of lyrium, the blight then added to his creepiness.
You can see that by how red lyrium affected the templar vs how blight affects.. whoever contacted it. Blight doesn't cause mutation, it creates a ghoul, gives the contacted person a sickly looking skin and cataract-like eyes. Lyrium just grows out of people (why? It makes me think about radioactive mutations from uranium, lyrium looks a lot like Torbernite
. Lyrium does explode, makes me think of atomic bombs [uranium isn't explosive, obviously, but it's still amusing to me]), and it was already been said that the ancient magisters underwent physical mutation because of large dosage of lyrium.
The magister lords of the Tevinter Imperium were widely reputed to have been so affected by their years of lyrium use that they could not be recognized by their own kin, nor even as creatures that had once been human.
This makes you ask - what does the blight do to lyrium? Maybe it just reallyyyyyyyyyy enhances it's powers, lyrium is dangerous on it's own, if you increase its' power tenfold it would be deadly. Why does lyrium, if so, reacts to blight in that manner?
And what is the taint? There is what you discover in the Warden's Keep DLC, [which I remember only vaguely so] I took an excerpt from the DA wiki:
The darkspawn taint also carries magical power equivalent to lyrium, and allows darkspawn emissaries to cast spells. In the Warden's Keep DLC, Avernus reveals that through his research, he has discovered a way to use the taint in Grey Warden blood to cast powerful magic without the use of lyrium or spirits. He tells that the taint holds greater power than known. Even non-mage Wardens can access this power, since all Wardens carry the darkspawn taint.
So... Taint has magical properties, this, in turn, enhances the lyrium, which is alive. Besides that, the taint seems to act like an accelerator of some sort - the darkspawn don't have healers, their wounds heal quickly. They can even sprout missing limbs given enough time. Crackpot theory: maybe what the taint does it accelerate time within a person? The sickly skin, the cataract, dementia, maybe they experience time faster, which causes them to grow mad? (Makes me think of Zero: Virtue's last reward)
This makes me think Cory ingested the red lyrium himself in order to enhance his powers and, also, that lyrium doesn't get infected that easily by the blight (Cory had the blight for thousands of years, his lyrium shards retained their blue-ness throughout the entire time). Perhaps he needed to consume red lyrium so he'll be able to unlock the orb Solas gave him.
The farther one is from the fade, the better he deals with raw lyrium. Mages that come in contact with raw lyrium will die, this is probably because the raw lyrium enhances one's abilities so much that he can no longer contain them... Cory's measured doses of non-raw lyrium for a long duration of time made him an powerful abomination, enhancing his.. magic.. circulation.. whatever, integrating lyrium to him bones, and making him ageless (like the elves were in the time of Arlathan).
I haven't read the calling but
Do you think that the fact the the magisters are (sort of).. Immune.. to the taint is because they consumed massive amount of normal lyrium?
Cory is clear minded & eloquent & he can 'speak' the calling... Maybe not unlike the Elvhen gods? Mythal can body hop as well, maybe Cory was closer to being a god than he imagined, and if so lyrium is the key for that (he was nothing more than a powerful mage at the beginning of his time), even if it made him an abomination in the process.
Lyrium sings afterall, so a person who ingests heavy doses of lyrium (through a long period of time) and gets the taint... Can sing into the mind of the tainted. Makes perfect sense TBH, especially since the taint connects the mind of the darkspawn.
Cory didn't control the 'singing' at first, he 'sang' in his slumber. That strengthens what Bianca said about lyrium being alive (in some manner), a non living organism can't become part of someone and function on its' own, so lyrium must be alive, but where does it come from? What gives it life? What gives it the ability to sing? Through what does it sing? Maybe it sings directly to the souls of people?
Crackpot theory: perhaps lyrium is the fade manifested in the 'real' world, it holds its' connection to the fade, so it speaks to the souls of people. Maybe it is heard as a song because it speaks to the soul, and not to the body/mind of the person, and if you were more connected to the fade/to your spirit you'd understand its' song.
Maybe all this means that since lyrium connects one to the fade more easily then those connected to the fade can deal/are immune to the taint? This, in turn, means that a person who is connected to the fade can connect with other's spirits more easily, read them, manipulate them, 'sing' to them.
Also! Darkspawn search for the old gods, they 'sing' to them, not unlike Cory. So wouldn't it mean that the old gods are already tainted? Maybe the only thing the darkspawn do is awaken them, like Hawke awakened Cory. Perhaps the old gods were imprisoned, like Cory was. Cory's like the old gods in more ways - the archdemons can body hop, they body hop into another tainted being if killed.
So following this line of thought - we have the Elvhen pantheon, some sort of mages with extraordinary connection to the fade, this includes... the Forgotten Ones.
The Forgotten Ones being the Old Gods is something that has been brought up here a lot, and it makes sense to me. While there may be only 3 Forgotten One names that are known, there were more (they are called forgotten for a reason). So.. If the Forgotten Ones had the connection to the fade like their counterparts, they would be able to 'sing', and if they are tainted.. Well... They could sing to darkspawn. Since the Old Gods can speak to dreamers, and the Forgotten Ones are the Old Gods by that theory, it suggests it's because of their strong connection to the fade, and that the 'song' can be interpreted when another is connected to the fade.
Super crackpot theory:
Since it seems like the Forgotten Ones are aspects of the Elvhen pantheon, then, perhaps, they are great dragons (ancient, powerful dragons) on which soul parts of the Elvhen gods were forced upon (or not forced, maybe they made a deal). They were also the source of the gods power, both allowing them to body hop, as well as giving them power.
The blood of Great dragons can confer enormous power. According to Aurelian Titus in Dragon Age: Until We Sleep, the Dreamers of ancient times commanded The Fade. The last Tevinter magisters were close to reclaiming this power, and "All they needed was the blood of the Great dragons."[2] He further states, "With it, I have tapped the power of gods."
Perhaps, even, the Forgotten Ones were entirely controlled by the Elvhen pantheon, and were there to keep the people in check, terror is a great way to scare someone to submission... And it could be untrue as well - the great dragons are not high dragons, perhaps they resisted the control of the Elvhen pantheon and wage war against them because they want to get entirely free from their geas.
As for Andruil, who wanted to hunt the Forgotten Ones, we already know she was nuts, but what if her reason for hunting the Forgotten Ones is to make the other gods fall? If she kills the Forgotten Ones the gods lose their powers.
The reason I think all my rambling are reasonable is because the voices of the well managed to recognize the fact Cory uses his dragon to allow him to body hop, it's plausible they were aware of that because they knew such thing exists. The voices are the will and knowledge of priests of Mythal, they aren't all knowing, there is a reason for them to say with such certainty that Cory's dragon needs to go down.