Rust blade

A rust blade was a rare type of magical dagger that had the constant effect of automatically rusting any metal with which it came into contact. Any other enchanted, metal items had a chance to resist this effect, depending on their enchantment level, and total number of special powers or spell-like abilities

Due to their constant effect, these daggers could not be kept in metal scabbards or sheathes and were potentially a hindrance to its wielder or allies.

Effects on rust monsters
The crafting of a rust blade required the meat from a rust monster's antennae. Due to this, if rust monsters smelled the weapon, they would attack and attempt to devour it over any other nearby metals. If the monster even touched the blade with their antennae, it would dissolve into a hot pile of metallic goo the monstrosity would find irresistible and nourishing.