Dreams of the Dying Land
The Mist is a terrifying result of the Catastrophe or the years soon after. Scholars argue whether the mist merely spawns monsters, if it creates them or summons them. In general, the closer you get to it, the more supernatural the perferred explanation is. Regardless, great alien creatures are known to be within the mist. These creatures are hostile and hungry.
The mist is a world wide phenomena. Any area 200 feet or less above sea level will generally be covered by the mist. It also rises and falls as normal fogbanks would. Stories say that once is a great while, it will behave as if controlled by a greater intellect and rush over areas normally safe, bringing great devastation.
To an outside observer, the mist normally looks like a regular fog bank of great thickness, occasionally large shadows can be vague perceived. Inside the mist, visibility drops to near zero and sound is muffled and echoes oddly.