Plaresh

Medium Fiend chaotic evil

STR	DEX	CON	INT	WIS	CHA
2 (-4)	17 (+3)	16 (+3)	6 (-2)	12 (+1)	3 (-4)

Skills:
Damage Immunities: poison
Damage Resistances: cold, fire, lightning; bludgeoning, slashing, and piercing
Condition Immunities: charmed, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned
Senses: blindsight 30 ft. (blind beyond this radius), tremorsense 60 ft., passive Perception 11
Languages: understands Abyssal but can't speak

Grinding Maws: The plaresh can burrow through harder substances such as wood, stone, or even metal. While doing so its burrow speed is reduced to half, and it creates a cluster of bore holes that leaves the material porous and weak. The material has -5 to its AC and half the usual hp.
Magic Resistance: The plaresh has advantage on saving throws against spells and other magical effects.
Swarm: The plaresh can occupy another creature's space and vice versa, and the plaresh can move through any opening large enough for a Tiny worm. The plaresh can't regain hp or gain temporary hp.

Bites: Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm's space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer. The target must make a DC 13 Constitution saving throw, taking 7 (2d6) poison damage on a failed save, or half as much damage on a successful one. If the target is wearing nonmagical armor, the armor takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 is destroyed.
Infest Corpse (Recharges after a Long Rest): The plaresh targets one dead humanoid in its space. The body is destroyed, and a new plaresh rises from the corpse. The newly created plaresh is free-willed but usually joins its creator.