These strange, normally docile creatures corrode ferrous metals, then gobble up the rust they create. In doing so, they have ruined the armor, shields, and weapons of countless adventurers.
Rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume. They ignore creatures not carrying such metals, but can become aggressive toward those bearing steel weapons and armor. A rust monster can smell its food at a distance, immediately dashing toward the scent’s source to corrode and consume the object.