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 SonsSpawn of Kyuss were created when Michael Mcdonald, a reader of White Dwarf Magazine 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 1977 Monster Manual. Advanced Dungeons & Dragons would flesh this out (or, in) in "The Ecology of the Rot Grub" Dragon Magazine #122 (June 1987). Second Edition carried this to the Monstrous Compendium Volume Two (1989) and then in the Monstrous Manual (1993), both times under the "Worm" entry. 3e didn't get around to it at first, such that it became a Tome of Horrors opportunity; much later in 2007, Hasbro restored it, in singular and swarm versions, for Dungeonscape. This would set a precedent; it likewise wouldn't show up in 4e until the Monster Manual 3 and the Book of Vile Darkness, and was delayed until Volo's Guide to Monsters to make its 5e debut.
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Pathfinder
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5e Grubs infested Zombie