RUST MONSTER

Rust monsters inhabit only dark subterranean places. They roam such places in search of their food — metals of all sorts, but principally ferrous based metals such as iron, steel, and steel alloys (such as mithral and adamantite arms and armor). 

If the rust monster touches the metal with its two antennae (roll "to hit" die) it rusts or corrodes the metal. 

Note that magically endowed items gain a saving throw, a 10% chance of not being affected for each plus, i.e. a +2 weapon or armor gains a 20% chance of being saved. Metal affected rusts or corrodes and immediately falls to pieces (which are easily eaten and digested by the creature). 

Weapons striking a rust monster are affected just as if the creature's antennae had touched them. 

Rust monsters can smell metal at 9" distance. They will stop for a melee round to devour such items as a handful of iron spikes or a mace if a fleeing party throws them away, but they will go after ferrous metal in preference to copper, silver, etc.