40 Rust Monsters
Disclaimer: we are not doctors and Freeman’s brief research into the connections between Rust and Tetanus are not to be considered valid medical advice! It's hard to believe this little moth, tick, bedbug weirdo came at us from tall the way back from Dungeons and Dragons 1e and that it's design hasn't really changed all the way up to 5e.
That being said…interesting stuff! A simple episode for a simple creature, but with no less flavour than usual! Enjoy!
Draconomicon: Book of Dragons - WotC
Planes of Law – WotC
Dragon Magazine Issue #169, pg. 92 – TSR, Inc.