From what I understand, they're magisters from Tevinter who trespassed upon the Maker's golden city and were punished by becoming tainted with the Blight. It is a human's tale, but it's the closest I have to a story on their origins.

[She didn't get it exactly right, but it's the best that she knows.]

What is important is that they are dangerous to everyone on Thedas. The Hero of Ferelden, another elf, managed to push them back ten years ago, but they still remain a threat to this day.

When I return home, I fully intend on stopping at least one facet of that threat by destroying Corypheus and mending what he's done.