The fog first appeared in 301 AF, after an attempt by Sartan to seize the realm of Valnurana for Himself. A Divine alliance struck down the God of Evil, imprisoning Him beneath an overturned mountain deep within the Sefyric Ocean. At this point, a mysterious red fog began to seep from the mountain, and with it came a variety of vicious beasts and daemons.
The fog was revealed to be the very essence of Sartan, and His followers began to impale themselves upon a stalagmite found on the mountain, their self-sacrifice fueling the strength of the fog and enabling it to spread further each day.
As the fog encroached upon the mainland of Sapience, it became clear that its effects were indeed toxic, as it exterminated the plantlife in every room it spread to, from the Savannah to the Black Forest and beyond.
Eventually a way to combat the fog was discovered, by filling a sacred decanter with the essence of the daemons and beating it back little by little. While Oakstone, the Church, and many others fought against the fog in this manner, Sartan found the strength to return from his imprisonment, forming a new body. The fog was called back to the island from whence it came and remains contained there in a somewhat latent state.