Black Powder Form. The horror can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing. 

Innate Spellcasting. The horror innate spellcasting ability is Charisma (spell save DC 13, +.5 to hit with spell attacks). It can innately cast the following spells, using its black powder form but nothing else as components. 

At will: produce flame, firebolt

2/day: scorching ray, fireball

1/day: conjure elemental (fire elemental only), wall of fire

Ignite. If the Powder horror takes fire damage, part of its body ignites in flame, the Powder Horror takes double damage and all creatures within 5 feet must make a DC 11 Dexterity saving throw or take 3(1d6) fire damage. 

Death Burst. When the horror dies, its black powder form ignites with flame, any creature within melee range of it must make a DC 12 Dexterity saving throw, at disadvantage if the creature was engaged in melee combat with the powder horror, or take 5(1d10) fire damage and catches fire on a failed save, or half as much and does not catch fire on a successful save. A creature that has caught fire takes 5(1d10) fire damage at the start of each of its turns, until someone takes an action to douse the fire. 

Multiattack. The Powder Horror makes 2 slam attacks

Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage. 

Powder Gust. Each creature that enters the powder horrors space must make a DC 14 Strength saving throw. On a failure, a target takes 13 (2d8+4) bludgeoning damage. If it is large or smaller, it is also grappled (escape DC 13). Until this grapple ends, the target is restrained and unable to breath. If the saving throw is successful, the target is pushed out of the Horror's space. At the start of each of the Horrors turn, each grappled creature takes 13 (2d8+4) bludgeoning damage. As an action, a creature within 5 feet of the Horror can pull a creature out  by succeeding a DC 13 strength check.