Plaresh

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.