Huge elemental, chaotic evil
Armor Class 14
Hit Points 68 (8d8 + 32)
Speed 0 ft., fly 60 ft. (hover)
Abilities Str 13 (+1), Dex 18 (+4), Con 18 (+4), Int 5 (-3), Wis 10 (+0), Cha 5 (-3)
Damage Resistances lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities poison
Condition Immunities exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses darkvision 60 ft., passive Perception 10
Languages understands Infernal but doesn’t speak
Challenge 5 (1,800 XP)
Dense Air Form. The vile fog can enter a hostile creature’s space and stop there. it can move through a space as narrow as 1 inch wide without squeezing. The area inside the vile fog is heavily obscured.
False Appearance. While the vile fog remains motionless, it is indistinguishable from a normal cloud of fog.
Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.
Choke. The vile fog targets one creature in the same space as it. The target must make a DC 15 Constitution saving throw. On a failed saving throw, the creature takes 9 (2d8) bludgeoning damage and is suffocating until the end of the vile fog’s next turn. On each subsequent turn, the vile fog can use its bonus action to continue to choke the creature so long as the creature remains in the same space as the target; the creature takes another 2d8 bludgeoning damage and continues to suffocate. On the creature’s turn, it can repeat its saving throw, ending the suffocating effect on itself with a success.
Abjuration Absorption. Whenever a creature within 30 feet of the vile fog casts an abjuration spell of 6th level or lower, the vile fog can disrupt the spell. When it does, the spell fails and the vile fog regains a number of hit points equal to 1d8 times the spell’s level.