Foul and corrupt are you
Who have taken My gift
And turned it against My children.
The ancient Tevinters did not originally consider blood magic a school of its own. Rather, they saw it as a means to achieve greater power in any school of magic. The name, of course, refers to the fact that magic of this type uses life, specifically in the form of blood, instead of mana. It was common practice, at one time, for a magister to keep a number of slaves on hand so that, should he undertake the working of a spell that was physically beyond his abilities, he could use the blood of his slaves to bolster the casting.
Over time, however, the Imperium discovered types of spells that could only be worked by blood. Although lyrium will allow a mage to send his conscious mind into the Fade, blood would allow him to find the sleeping minds of others, view their dreams, and even influence or dominate their thoughts. Just as treacherous, blood magic allows the Veil to be opened completely so that demons may physically pass through it into our world.In Dragon Age Origins:
The rise of the Chant of Light and the subsequent fall of the old Imperium has led to blood magic being all but stamped out—as it should be, for it poses nearly as great a danger to those who would practice it as to the world at large.
—From The Four Schools: A Treatise, by First Enchanter Josephus.In Dragon Age II:
Thought for some time to have been stamped out in the Imperium, the practice of blood magic seems to be on the rise in recent years. The current Black Divine has lifted most of the prohibitions against dreamwalking, and so many mages in the Imperium now use it openly in the name of research.
—From The Four Schools: A Treatise, Third Edition, by First Enchanter Josephus.