Plague / Epidemic
A deadly disease originating from the north arises. It devastates almost all giants and goliaths, and many human and elven lives are lost as well.
A disease carried through the fish from the sea spread a plague of decay throughout the lands of the North. Though it doesn't affect the humanoids, it culls the wildlife and plants in the region, causing widespread famine across the north. The plague was however weak to temperature, and did not spread beyond the cold lands. The famine lasted over 400 years, and not many were left when the plague was done.