In retrospect I think these would work better as a hazard than an actual creature. They didn’t get much flesh in my game. My players were suspicious of the “column of flesh with puckers” and attacked the column at range from outside the room. After the swarm spilled out they cast wall of fire and left them to burn (or flee).
Armour Class 14
Hit Points 160 (40d6)
Speed 20ft., climb 20ft.
Initiative Speed +7 (Dex +2)
STR 3 (-4) DEX 13 (+1) CON 10 (+0)
INT 1 (-5) WIS 7 (-2) CHA 1 (-5)
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, paralyzed, petrified, prone, restrained, stunned
Senses blindsight 10ft., passive Perception 8
Challenge 7 (2,900)
Infestation. Any living creature that takes damage from a rot grub swarm becomes infested unless it succeeds at a DC 15 Dexterity save. On a failed save, the infested creature gains 1 level of exhaustion per round as the rot grubs burrow through and consume its flesh—this effect continues as long as the victim remains in the swarm and continues for 1d6 rounds after it leaves the swarm. Any energy-based attack that deals at least 5 points of damage to the victim automatically destroys all of the rot grubs infesting it, ending the effect prematurely. Additionally, any effect that removes disease instantly ends a rot grub infestation. Immunity to disease offers no defense.
Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. The swarm can’t regain hit points or gain temporary hit points. Additionally creatures sharing the same space as a swarm are distracted. They gain disadvantage on all attack rolls and ability checks (but not saving throws).
Bites. Melee Weapon Attack: +6 to hit, reach 0 ft., one creature in the swarm’s space.
Hit: 15 (5d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer. The creature is also subjected to an infestation attack.
This nauseating carpet of wriggling white grubs undulates outward in a pallid wave of hunger.