The Fog

Cosmology
As stated by ThePinkPanzer, the Fog was a part of the cycle of death and rebirth before the Godherja. When people died, their soul went back to Aersanon and briefly stayed in the Fog before Aersanon sent the soul back into Aeras in some form. In those times, the Fog was much gentler as it didn't hold all collective human life and suffering. Following Aersanon's destruction, human souls simply stayed in the Fog as They were not alive to take the souls back to Aeras. The Fog was unleashed on the world because the part of Aersanon's mind holding it split open in The Frodbrokna.