Blood Magic

Transfigurations 18:10 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 Tevinter 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.

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. Thus, spells such as those that can influence dreams have been lost in the fall of the Imperium and only the likes of the Somniari have this power (though they do not need blood nor lyrium to enter nor control the Fade nor dreams). It is possible that the knowledge of this blood magic still exists, for instance the old god cult Primoris Incendia uses blood magic in a very different way to modern maleficarum. The elves also used and use blood magic differently, so we cannot assume that all knowledge is lost forever nor that the Chantry's spin on blood magic is entirely accurate.