^Wow, that makes a whole lot of sense. I considered that red lyrium might be diseased or rotting normal lyrium, but it never occurred to me that the Void might just be titan corpses. That still doesn't explain how the taint got separated from red lyrium, or why the Old Gods would want the Magisters Sidereal to enter the Black City.
Perhaps there's red lyrium in the Black City, and it became the taint when it got shoved into the Fade. As for the Call, I have a theory that it doesn't actually come from the Old Gods, but from the taint itself. It wasn't the Old Gods that spoke to the magisters - it was something, or somethings in the Black City. Cole talks about the red templars hearing whispers that come from a door that wants to be opened...
Solas says that the Evanuris are in the Fade - I took this to mean physically inside it. I think they're actually in the Black City, which used to be Arlathan. Before Solas made the Veil, Arlathan was like the Shattered Library. It was partly in the Fade, and partly in the physical world. After he made the Veil, some or all of it got shoved into the Fade. When that happened, the Evanuris went into the Fade with it.
The whole reason Solas made the Veil to begin with was to trap them in the Fade. If just their spirits/minds where there, they could just come back. Dreamer mages can enter and leave the Fade mentally at will. It's physically entering and leaving that's nearly impossible. Arlathan was the capital of Elvhenan, so it makes sense that that's where the Evanuris would be.
An alternative explanation is that the red lyrium was actually still confined until Cory & crew got there. Then they accidentally on purpose unleashed it, blighting the entire place. That wouldn't explain Cory's instance that city was already blighted, though. It's more likely that the "Golden City" was an illusion created by the Evanuris, who tricked the magisters into entering the city so that they might free them.
I find it very suspicious that Cory had his own Call but Kieran, who can have Urthemiel's soul, doesn't. If the Call really comes from the Old Gods, why doesn't he have one? It could be because he isn't a dragon. But that wouldn't explain how Cory gained a Call. The fact that a tainted creature without an Old God's soul can have a Call suggests that the Call doesn't come from the Old Gods, but from the taint itself.
I think that it was actually the Evanuris that spoke to the magisters. They pretended to be the Old Gods. And that's still what they're doing. They're the Call that tainted creatures hear.