Monster Monday - Rust Monster

Art from Fat Goblin, used under license

Never doing damage, the rust monster still strikes terror into the heart of the bravest warrior. Instead of biting and clawing, the monster rusts weapons and armor away. The rust monster is a creature that many of us grognards cut our teeth on. My earliest memory of this creature is from Frank Mentzer's Basic (Red) D&D boxed set. It featured in the solo adventure in the Player's Guide.

Rust monsters are almost as old as the game itself. The rust monster first appeared in the Greyhawk Supplement for OD&D. Gygax said: "RUST MONSTERS: These seemingly inoffensive creatures are the bane of metal with a ferrous content, for as their name implies they have the effect of rust upon such substances, and this happens nearly instantaneously. Any hits by or upon a Rust Monster cause even magical weapons to rust and fall to flakes. Armor is affected in a like manner. The creature is very fast, being attracted to the smell of the iron-based metals, and when alone it will devour the rust it has caused." (Greyhawk, pg 39.) They have appeared in every edition of the game since: Mentzer's Basic (mentioned above), Monster Manual (AD&D), Monstrous Compendium (AD&D 2E), Monster Manual (3 and 3.5), Monster Manual 2 (4E), and Monster Manual (5E).

Here are the Swords & Wizardry stats for rust monsters:

HD: 5

AC: 2 (17)

Attacks: 2 antennae

Save: 12

Alignment: neutral

Number Encountered: 1-2

CR/XP: 5/240

Rust monsters are fast, but not deadly by themselves. In different editions they wavered between eating ferrous metals only and eating any metal (gold, silver, etc.) To best challenge a party with rust monsters, they need to be paired with something else. 

The great wizard Peln Fearlash wished solitude. To facilitate this, he created a trap at his front door. Three harpies reside in the antechamber. Their crooning activates a sliding door that releases rust monsters into the room.

An anis hag deep in the forest keeps a rust monster as a pet to get rid of the pesky warriors gear that she cannot cook. 

The ogres in Darkmoon Keep breed rust monsters and release them into the forest around their keep. They have not mastered metallurgy, but have found a way to even the odds.

The man-apes of the southern jungles worship a mammoth rust monster as a god.

Giant rock weasels have made their burrow inside of a rust monster carcass. This close contact has caused their claws to gain the ability to rust metal, just like a rust monster.