On the Eyes of Midnight
Strahd's Choking Fog
Legend says that Count Strahd is able to cause the choking mist around his castle and the village of Barovia to spread out over the entire domain. The vapor infuses itself around a the vital organs and acts as a latent poison.
Once a victim has entered this fog, it is said that they can never leave Barovia without express permission from Strahd or an antidote. Prolonged exposure to the fog can cause permanent damage and quick death.