Killing Cloud

A lethal poisonous cloud of gas that burns lungs and melts flesh flows out from fissures, an alchemist's lair, or the pyroclasmic aftermath of a volcano to sweep over the area. Extraplanar explorers know that these are a particularly common threat when traveling in the lower realms, sometimes covering entire regions in a miasma of flesh-eating death.

Fogged Vision. The maximum range of any sight-based senses is 5 feet. In addition, Perception checks are made with disadvantage , and all passive scores (including passive Perception) are reduced by 5.

Poison Cloud. At the end of every hour in the area or when exiting the killing cloud, a creature makes a Constitution saving throw or takes 45 (10d8) poison damage. A creature that is immune to poison damage or does not need to breathe automatically succeeds on this save. Any creature that takes 80 or more poison damage from the cloud becomes poisoned , and a poisoned creature that fails its saving throw suffers one level of fatigue . On a critical failure, a poisoned creature falls unconscious for 60 (1d12 × 10) minutes. 

Realize Danger. Any adventurer that makes an Arcana or Nature check realizes the nature of the poison cloud and its effects.

Spell Solve. The fog can be temporarily blown away by the casting of a gust of wind spell or other wind-based magic that uses a spell slot of 6th-level or higher, giving the adventurers advantage on saving throws and checks made against it.

Spoiled Supply. The poison cloud ruins 1 Supply each hour, spoiling rations and contaminating water. 

Up and Away. The use of levitate , fly , or similar magic is an automatic success in the group check for that adventurer.