Studies of the Veil have never been thorough. The Tevinters once theorised that the Veil is thin in places that have seen great bloodshed. As the Chantry is so fond of reminding us, the magisters of the Imperium only once crossed physically into the Fade, the catastrophe that blackened the Golden City. As the story goes, the ritual consumed a vast quantity of lyrium and the blood of a hundred slaves. But was it the deaths that bridged the worlds, or simply the blood and lyrium? Or both?
Demons seize every opportunity, every tear in the Veil, to enter our world. Once the Veil is torn, it is extremely difficult to mend, some say impossible.
--From The Lectures of First Enchanter Wenselus