Rust Monster

About 5 feet long and 3 feet tall at the shoulder, the rust monster has lumpy, rough hide shading from yellowish tan below to rusty red along the back. Two prehensile antennae hang beneath its eyes, and its armored tail ends in a bony knob shaped like a two-bladed paddle. It smells of damp, corroded metal.

Normally placid, the creature grows frantic once it scents metal, which it can do at 90 feet, and rushes straight for it. A successful attack roll means its antennae touch metal, which instantly rusts or corrodes and crumbles into edible flakes. Metal weapons that hit it suffer the same fate. Magical items resist with a 10% chance per plus (20% for a +2 sword or suit of armor); a magical metal item that is not a weapon is treated as +2 for this check.

Ferrous metals such as iron, steel, mithril, and adamantite are its favorites, far ahead of copper, silver, gold, or platinum. Toss a handful of iron spikes or a mace behind a retreating party and it stops for a round to eat; drop copper coins and it keeps coming in hope of armor and swords. Even mid-fight there is a 30% chance it pauses a round to feed, and every meal takes exactly one round whatever its size.

Rust monsters keep to caverns and dungeons, usually alone or in pairs, with a 5% chance of a single youngster that fights at half strength but eats like an adult. They wander whole underground complexes hunting metal, preferring worked and forged pieces over raw ore much as a person prefers clean water to swamp water. Their only treasure is loose gems, mostly the kind that once decorated armor and sword hilts. Many mages report being sniffed once and then ignored. Dwarves and gnomes, being miners and smiths, hate them and will go to any length to be rid of them.

They clean metal debris out of underground strongholds, and some share a lair with a corpse creeper in a tidy partnership: the creeper eats the organic refuse while the rust monster eats the metal.