Rot grubs are nearly mindless creatures that exist primarily in swarms, a stat block for which is provided in Volo’s Guide to Monsters. They’re a nasty surprise for any adventurer who stumbles across them, since the only way to fight them off once they’ve burrowed into you is to intentionally burn yourself. However, to talk about their having “tactics” is to give them too much credit, and I’m only bothering to write them up because a reader requested it.
A swarm of rot grubs is easy to hit, having an armor class of only 8, but not so easy to destroy. The swarm is resistant to piercing and slashing damage, reflecting the fact that you can kill a lot more grubs with a broad bashing weapon than with a thin cutting edge or poking point. The swarm is also immune to most debilitating conditions, stunned and unconscious being two standout exceptions. The swarm can be blinded, but that doesn’t mean much, since it has 10 feet of blindsight. Except for its Constitution, which is average, the swarm’s ability scores are pitifully low.
The swarm has only one attack, a bite that does only indirect damage. Rather than take place at the moment of attack, the damage occurs at the start of the target’s turn, and it varies in proportion to the number of grubs that have burrowed into the target’s flesh (1d4 per hit). The target must cauterize the bite wound on this turn, or the burrowing grubs will do continuous round-by-round damage until the target is either cured of the infestation or dead.
A swarm of rot grubs moves at 5 feet per round across any surface, horizontal or vertical, and it lacks the Intelligence to do anything more complex than move directly toward the first other living being that comes within 10 feet of it. But I’d allow one exception to this, for the dungeon master who likes to play dirty: Because they can climb, rot grubs can swarm across a ceiling and drop down onto someone who walks beneath them. The swarm has no self-preservation instinct (with such a low movement speed, even if it had one, it wouldn’t do it any good), only the instinct to feed.
A rot grub can be encountered singly, but it doesn’t get an attack; instead, the target makes a DC 10 Dexterity saving throw when it enters the rot grub’s space or vice versa.
And that’s pretty much all there is to say about rot grubs, but I’ll leave you with this haunting thought: Rot grubs are maggots. Maggots are larvae. Larvae are the immature forms of holometabolous insects. Therefore, eventually, rot grubs metamorphose into something else. “This isn’t even my final form!”
Next: elite githyanki and githzerai from Mordenkainen’s Tome of Foes.