This war refers to the apocalyptic conflict humanity waged against the children of Asag at the dawn of civilization. The disparate Lords of Men united under a single banner to drive the children of Asag out of Midworld, and into the Northern Wastes. Monsters of Midworld, such as the Colossi and the Brood Mother's dragons, were all either slain or banished beyond the edge of the world as a result of this conflict.
The exact details of the Great War are hotly debated among sages at the Library of Ambion. Perhaps the only trustworthy records are those maintained in the Magic University in Muraius.