Encounter Element: Dread Fog
Living creatures treat an area of dread fog as , making sight-based Perception checks with disadvantage. The whispers of the dead also make it difficult to discern sounds, and hearing-based Perception checks are made with . The sudden chill in the air extinguishes nonmagical open flames. A creature with Sunlight Sensitivity ignores that trait while in an area of dread fog. An area of dread fog can be temporarily cleared for 1d4 rounds by casting .
Source