Rot Grub
Generic Hazard
Rot grubs are finger-sized maggots that eat living and dead creatures, although they can survive on vegetation. They infest corpses and piles of decaying matter and attack anyone that disturbs them. After burrowing into a creature, rot grubs instinctively chew their way toward vital parts.
Rot grubs recoil from flames, and fire is the main weapon against rot grubs once they’re inside a body. Magic that neutralizes poison can also stop them before they kill their host.
Single Rot Grub
Rot grubs pose a threat both singly and as a swarm. See the stat block for the mechanics of a swarm of rot grubs. A single rot grub has no stat block.
Any creature that comes into contact with a single rot grub must succeed on a DC 10 Constitution saving throw or be poisoned, as the rot grub burrows into the creature. The poisoned creature takes 3 (1d6) poison damage at the end of each of its turns. Whenever the poisoned creature takes fire damage, the creature can repeat the saving throw, ending the effect on itself on a success. If the poisoned creature ends its turn with 0 hit points, it dies, as the rot grub kills it.
Source: Volo’s Guide to Monsters p. 208, Mordenkainen Presents: Monsters of the Multiverse p. 237