Ash Wraith

Ash Form. The wraith can enter another creature's space and stop there. Whilst in medium or larger creatures space the wraith has half-cover. It can move through a space as narrow as 1 inch without squeezing.

Unusual Nature. The wraith does not require air, food, drink, or sleep.

Multiattack. The wraith makes two slam attacks.

Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 6 (1d8 + 2) bludgeoning damage plus 4 (2d4) fire damage. 

Ashen Possession (Recharge 6). A creature in the wraiths space must make a DC 13 Dexterity saving throw or take 9 (2d6 + 3) fire damage. At the start of the targets next turn, if the wraith is still in the targets space, the target must make a DC 13 Dexterity saving throw or take 9 (2d6 + 3) fire damage and is grappled. Ability checks made to end the grappled condition are made at disadvantage. At the start of the wraiths next turn, if the target is still grappled, it must make a DC 13 Charisma saving throw or be possessed by the wraith. If the target is not grappled but still in the wraiths space, the target must make a DC 13 Dexterity saving throw or take 9 (2d6 + 3) fire damage and is grappled.

A creature possessed by the wraith is incapacitated and loses control of its body. The wraith now controls the body but doesn't deprive the target of awareness. The wraith can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to the charmed condition. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

The possession lasts until the body drops to 0 hit points, the wraith ends it as a bonus action, or the wraith is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the wraith reappears in an unoccupied space within 5 feet of the body.