Rot Grub

Rot Grubs are a species of voracious, predatory annelids native to the worlds of Dungeons & Dragons. They were born when Gygax realised that there were no hiding places too foul or disgusting for his players to overlook in pursuit of treasure, so he came up with a Gotcha Monster just to punish them. Rot Grubs are maggot-like scavengers that infest offal, dung, carrion, and similar masses of decomposing organic matter; when disturbed, they will launch themselves at living flesh, burrowing rapidly inside and then chewing a quick but painful route to their host's heart. Unless burned or cut out (which could kill you), they would reach the heart and then eat it within a number of rounds/turns, which would most certainly kill you.

Often considered one of the nastiest - and most embarrassing - ways to lose a PC, especially given how overpowered their abilities were in earlier editions of the game. The Spawn of Kyuss were created when somebody (probably Gygax) decided "hey, rot grubs are fun; but it'd be even more fun if anyone they kill then turns into a worm-infested wight who can infect the other players with rot grubs too!"

This nasty little bug first appeared in the original Monster Manual (1977) for Advanced Dungeons & Dragons 1st edition; it would be fleshed out in "The Ecology of the Rot Grub" in Dragon Magazine #122 (June 1987), and would be updated to 2e first in the Monstrous Compendium Volume Two (1989) and then in the Monstrous Manual (1993), both times being printed under the "Worm" entry. It wouldn't make an appearance in Dungeons & Dragons 3rd Edition until late in that edition's lifespan, showing up in both singular and swarm versions in 2007's Dungeonscape. This would set a precedent; it likewise wouldn't show up in Dungeons & Dragons 4th Edition until the Monster Manual 3 and the Book of Vile Darkness, and likewise was delayed until Volo's Guide to Monsters to make its Dungeons & Dragons 5th Edition debut.