A few days ago, I went in search of the Green Fog. When I found it, the fog just sort of appeared around me, there was no fog on the horizon, no warning I was close. As I got deeper into the fog, it just increased in intensity around me in every direction, until it was too thick to determine a direction.
I would like to suggest the fog be a visible barrier, more like a volumetric cloud. You see the fog before you enter, with items in the distance increasingly obscured. As you enter the fog, until you get too deep, it should be obvious which way is the thicker or thinner part of the fog.
I can think of several ways to accomplish this, I guess it depends on the limitations of the Xsyon game engine.