Endless Fog

Perfect time to coop up inside, close all windows and refuse to go out for any reason.

Why would anyone be scared of a fog? Sure, it is inconvenient and a bit chilly but nothing terrible - and it usually passes in a few hours. What if both of these statements were not true anymore?
A fog so dense you can barely see your own body, so heavy you can physically feel it surrounding you, so vast it covers entire countries and regions, so long it may take weeks or even months until the fog is finally gone. Wherever it passes, all life stops and economy grinds to a complete stop. It spreads with enormous speed, faster than even the scariest amongst the ocean's killer waves. For many, the experience proves so scary and traumatic they either go insane or find salvation in religious fanatism.

Localization

Most of the records and evidence I am familiar with points to the theory that the World Fog comes from atop the Misty Falls. In fact, it is from the Misty Falls' significance in how many perceive the world that comes the phenomenon's other name - Otherwordly Fog. Something like that does not belong to our world, and has no right to be here. A vile and cruel sorcery, that is what it is!
Once a World Fog is unleashed, it rapidly spreads through the land. What is interesting, the Hummish legends speak of the calamities brought by the World Fog in much harsher and dramatic terms than the Kosari records. While many discard this as merely a love for exaggeration so common among the Hummish bards and chroniclers, I personally take a liking to the theory that something - or someone - causes the lands of the Ardenian Empire to be less affected by the fog.

History

Every century gets its own calamities, and it seems at least one of them has to be a manifestation of the World Fog. Some generations never get to see the sight, some live through several World Fogs. It is often taken by people to be a sign from above, and sometimes leads to entire nations falling apart.
The worst case of it was 156 years ago. Enormous Ardenian armies were marching to retake control of rebelled provinces, when a World Fog appeared. By the time it was gone, only a small fraction of soldiers have survived - lost, hungry, sick and scared. Such a calamity brought down the ruling dynasty and severely weakened the Empire's hold over its provinces, perhaps one of the most significant events leading to the Ardenian Empire's eventual degradation and fracturing.