The Great Cataclysm

A series of disasters - earthquakes, famines, cold winters, droughts, invasions of monsters and other phenomena that afflicted the Old Empire approximately one thousand years after its foundation.
One of the more widespread legends supposedly dating from this distant time is that the Empire was invaded by an army of undead led by the necromancer Cargobel the Icelord.
What contemporary accounts survive suggest that the Empire's population fell by as much as 90%.