The Great War Against Chaos was the cataclysm of 2302 IC, when the greatest Chaos invasion the world had yet seen swept out of the Chaos Wastes and very nearly extinguished civilisation in the Old World. United beneath the Everchosen Asavar Kul, tribes from Norsca and the eastern steppes marched south with daemons beyond counting, overrunning the northern realm of Kislev while the Empire — fractured by centuries of civil war and rival claimant emperors — seemed too broken to answer.
Salvation came from a priest of Sigmar named Magnus, remembered ever after as Magnus the Pious. Preaching unity in the founder's name, he shamed the feuding provinces into raising a single great army and led it north, joined by dwarf throngs from the Mountain Holds and a high elf host under Teclis of Ulthuan. The city of Praag fell and was warped into a living nightmare, but before the Gates of Kislev the allies shattered the horde, and Asavar Kul was slain.
The war remade the Old World's map and the Empire's soul. Magnus was crowned Emperor of a reunified nation, the Colleges of Magic were founded under elven guidance, and the watch on the north has never again been allowed to sleep.