MISTS OF RAVENLOFT

A deadly fog surrounds the lands of Barovia, imprisoning all creatures that lie within the realm. Even flying beasts are subject to this unnatural law. 

EFFECTS OF THE FOG

• A creature that starts its turn in the fog must succeed on a DC20 CON saving throw or gain one level of exhaustion (as per the PHB Appendix A). This exhaustion can't be removed while the creature is in the fog.
• No matter the direction travelled within the mists, or how far, a creature will always find itself back in Barovia.
• The area within the fog itself has the heavily obscured rule (as per the PHB Chapter 8).