
This is one of the codex entries that are translated after you drink from the well...Could the weapon created by Andruil be the taint?
I doubt it was the taint, or at least not the taint at that time. The weapon was glowing golden, not black or red. It could be what the Magisters where told they would find in the Golden City though: "we were promised the golden light, the power of the gods themselves". A golden glowing spear (of light?) created by an elven god...
Also, going by the Chant of Light, I'm starting to wonder if the Magister brought the taint to the Golden City as opposed to them contacting it there. That might actually explain why Corypheus had a change of personality right before the ritual happened, he was already transforming into a ghoul/darkspawn. This passage might be literal with Sin standing for Taint/Corruption:
And so is the Golden City blackened
With each step you take in my Hall.
Marvel at perfection, for it is fleeting.
You have brought Sin to Heaven
And doom upon all the world.
Maybe Red Lyrium pre-date the Taint/Blight and it was the female Magisters that went into the City who turned into broodmothers afterward and created the darkspawns...
Also, for crackpot theories, I'm starting to wonder if Andruil might have been Andraste. She gets the most references in DAI in term of elven lore beside Mythal and Fen'harel.