The haunting misasma is an undead creature that looks like a thick swirling inky black cloud. The cloud flows in a roiling mass engulfing opponents. Here are som stats for 5e.
This adventure idea uses the Haunting Miasma.
The haunting miasma appears as a thick black cloud. The cloud flows in a roiling inky blackness. It can flow through small gaps or spaces.
The haunting miasma occupies up to 4 adjacent squares.
The haunting miasma flows into the same square occupied by a character to attack engulfing the character. Squares engulfed by the haunting miasma are in darkness, like the spell. Characters engulfed are subject to an engulfing attack which does not require a roll to hit.
Armor Class: 8
Hit Points: 22 (5d8)
Speed: fly 15ft.
Damage Resistances: acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical weapon.
Immunities: necrotic, poison.
Susceptibility: The Haunting Miasma takes 2d6 from effects that generate a strong wind.
Condition Immunities: charmed, exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses: darkvision 60 ft., passive Perception 10
Life Drain special Spell Attack:
Any engulfed creature: 10 (3d6) necrotic damage. The target must succeed on a DC 10 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the creature finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
This image does not convey the blackness and darkness of the miasma. You win a few, and you lose a few, drawing is like that.