The First War Against Chaos. The newly founded Empire is attacked by the forces of Morkar the Uniter, Everchosen of Chaos. The war is desperate, short, and devastating. Eventually, Morkar is slain by Sigmar in a day-long battle that was said to have split the earth and rent the sky.
(
Canon Conflict: Other sourses say it was
Cormac Bloodaxe not
Morkar.)