You might be interested in this codex entry.
Information about Corypheus before he assaulted the Black City is still around the Imperium.
Master unveiled a new altar. It stands higher than a man, like a great statue, and great spikes jut out from its length, hungry for blood. Master calls it "the Claw of Dumat" and says that the altar will help bring Tevinter to glory. I praised it, as was expected, and Master smiled. It was good to see him smile again. He has been fearful of late, vexed by the loss of followers. He has met with the other priests, and in secret, I have heard them discussing ways to return the people of Tevinter to the ways of the Old Gods, as is only just. He spoke to me later in the day, and asked that I call him Corypheus, as it was the name he would take for himself after a ritual. Master - now Corypheus - told me that my people, the elves of old, were tied to the Fade, and that in order to carry out the will of Dumat, he would need to call upon the magic that lives in our blood. Corypheus told me to gather all of the elven servants and bring them to the western hall of our home at midnight. That is the hall where the Claw of Dumat is now kept. There are shackles across the top of the great altar, and pools lined with runes beneath the claws. I have sent my wife and children away, but have not warned the others. A few I may save. If I tried to save us all, we would only be killed in some other way, and others would die in our place.
Master once laughed and joked. He could be stern, but he was not a cruel man. The weakening of the temples brought fear into his heart, and that fear has changed him. The cuts upon his arms are deeper and longer where he used his blood magic more often. He speaks to his wife little. He listens only to the voices in his dreams.
It is almost midnight. The Claw of Dumat, great and spiked and merciless, is all my mind can see. I must gather the others. My family is safe. Corypheus will take me, but not those I love. - Words somehow preserved in blood at the statue's base