After The Maelstrom all Blood Mages or practioners of Blood Magic were rounded up and put to death. This was a significant portion of all the mages of the Mage Corp, who had been influenced by Magaras Thul to take up blood magic to improve their already impressive abilities. On finding that they had been made anathema in the sight of the Emperor many stood against the army holding them off for a number of weeks in a fortification that they had taken control of. In the end they were their own worst enemy consuming each others life force until they killed each other.
Others tried to escape to the far reaches of the empire but due to their addiction many were easily tracked down and destroyed. Rumors have come of others that escaped the purge but the official story is that htey have all been accounted for and destroyed.