Plague / Epidemic
"When the first Collosus appeared in the homelands of the Gnome people's it was devestating, such peaceful people were not prepared for the war that had walked unto their doorstep." --- Archmage of the North Tower
The Collosus sent to with the task of bringing destruction to the Gnomes arrives and does just that. The Gnomish folk are reduced down to 30,000 of their kind. A massive culling that they have never recovered from.