The fog was created during the Humbling to allegedly punish the mortal races for stealing the power of creation from the gods and using it recklessly. This magical fog is the distilled essence of planar magic. It comprises power from all the planes of existence, and it is able to produce miraculous feats of magic. However, the main goal of the fog is to destroy anything it comes across.
The fog can, for instance, instantly ignite any flammable material it touches. It can instantly sap the life from any organic being within its grasp, or perhaps it turns organic material into stone. The effects are limitless. Conventional wisdom is to simply run far away whenever the fog is sighted.
Some creatures, such as dragons, are immune to certain effects of the fog, but they are not wholly safe. The fog has been known to spontaneously summon deadly inhabitants from the other planes to rage against those of the Material Plane. One Orcish outpost was settled deep within the fog in the Mournlands since they are typically immune to the effects of the fog. They only lasted for a fortnight before a pack of howling creatures descended on them from the fog and ripped them to pieces.
Scholars across the land study the fog and try to devise tactics to stop its spread, for it is still spreading. The spread is slow, but whole sections of western Faerstice have been overwhelmed by the deadly fog. The orcs are on the forefront of this research, but they have few answers after over four centuries of study.