Maleficar

A maleficar is a mage who uses the forbidden arts, including, but not solely restricted to, blood magic. A known maleficar is slain on sight by templars.

An apostate is not automatically a maleficar, but many are. Bucking the authority of the Circle of Magi usually leads a mage into exploring the forbidden, especially if they feel they need such power to stay alive. This is common enough that many templars will simply assume that all apostates are also maleficar, or close enough to becoming one that it makes no difference.

Source
Dragon Age: Origins' Official Forums