A mysterious fog that surrounds the land of Barovia. Entering into the fog causes one to become immediately disoriented, although it is said that the Vistani are able to traverse it without being affected.
The fog seems to be different at Yester Hill; there Bell saw a vision of a white fortress on a hill above a great city and heard voices calling to him, urging him either to go further towards it or to turn back and escape.
The fog seems unrelated to the Burgomaster's plea to "encircle" Barovia, given that texts found in the Baron's library indicate that the fog is at least many hundreds of years old, and Strahd von Zarovich claims to have been there at its birth.
Although we were able to enter the grounds of Ravenloft with no trouble, once we tried to leave we found that we were prevented from doing so by a similar fog-like boundary.