A Whole Lot of Well, Shit
One of the things that used to puzzle me a little was how Corypheus even learned about red lyrium in the first place. The quest “Well, Shit” heavily implies that it was through Bianca, but that didn’t quite completely track for me. (Also, as much as I do not care for her, I also didn’t want to place the entirety of blame on her. I’d rather be upset with her for her failings in her relationship with Varric than making her a scapegoat for all the red lyrium shenanigans that came after. There’s always more at work behind the scenes in stories like this, anyway.)
This is the dialogue from “Well, Shit” - note that “Larius” can be switched out for “Janeka” depending on whom Hawke sided with in DA2 Legacy:
Bianca: *key found* “There you are.” *locks door* “They won’t be able to use this entrance again.”
Varric: “Bianca.”
Inquisitor: “You were the one who leaked the thaig’s location.”
Bianca: “It’s not like that. Not... entirely.” *sighs* “Shit.”
Bianca: “When I got the location, I went and had a look for myself. And I found the red lyrium, and I ...studied it.”
Varric: “You know what it does to people!”
Bianca: “I was doing you a favor! You wanna know how this stuff works just as much as I do. I just... wanted to figure it out.”
Inquisitor: “How did you go from studying red lyrium to giving the location to Corypheus?”
Bianca: “I found out that red lyrium... it has the Blight, Varric. Do you know what that means?”
Varric: “What, that two deadly things combine to form something super awful?”
Bianca: “Lyrium is alive, or... something like it. Blight doesn’t infect minerals, only animals. I couldn’t get any further on my own, so I looked for a Grey Warden mage. Blight and magical expertise in one, right? And I found this guy, Larius. He seemed really interested in helping my research. So I gave him a key.”
Varric: “Larius? He was the Grey Warden we met in Corypheus’s... aw, shit. I knew something seemed off.”
Bianca: “I didn’t realize until you said you found red lyrium at Haven. I came here and, well... then I went to you.
Inquisitor: “That name mean something to you, Varric?”
Varric: “He was at the Grey Warden prison where we found Corypheus. And he definitely wasn’t a mage before.”
What Bianca is definitely guilty of is giving Corypheus the location of a small buffet of red lyrium and an opportunity to study it. What’s not so certain is whether or not this also revealed red lyrium’s existence to him.
I believe it’s entirely possible that Corypheus could have heard about the existence of red lyrium before meeting Bianca thanks to another person he met in Kirkwall: Samson.
Unfortunately the same sort of codex entry does not show up in DA2/Legacy for Janeka, but this is a codex that is supposed to show (and is often bugged from not appearing): A Strange Sighting
Yesterday, I saw a man I could swear was Larius, just walking around Lowtown. I wasn't sure at first, but that armor, that hair... he's unmistakable.
He was talking to Samson, of all people. I can't imagine what a Grey Warden commander and an ex-templar vagrant have in common, but they seemed intent in their discussion. When I came closer, Larius pulled Samson into a doorway---I don't know if he saw me or not, but it seemed almost like they were avoiding me.
Anders says that no one as far gone to the taint as Larius would ever have been reinstated by the Grey Wardens. Even without Corypheus, Larius will be plagued by the calls of the Archdemons until they drive him mad.
I tried to contact the Grey Wardens, to find out why they let him return to Kirkwall, but they weren't talking. They won't acknowledge a Warden named Janeka, a prison in the Vimmarks, a captive darkspawn magister, or ever having worked with Father.
they didn't even want to hear what Corypheus claimed to be--I'm certain they know something about what happened down there, but whatever it is, they're not sharing.
--From the diary of Garrett/Marian Hawke
Samson, especially if you helped reinstate him near the end of the game, would have known about Meredith’s sword and related to Larius/Corypheus what it did for her, power-wise, before she became a red lyrium statue. It also helps explain how you can “reobtain” Meredith’s sword Certainty, which had definitely shattered and was what turned her, or helped her turn, into a red lyrium statue.
I will also note that Hawke trying to contact the Wardens about what happened and trying to get answers is likely what alerted Stroud/Alistair/Loghain and put them on the trail and being worried about the Archdemon-like body hopping. (Because personally, that too had seemed strange to me -- how they’d up and wonder and start asking in the first place. This way helps connect the dots.)