Rust monsters were subterranean scavengers known for their ability to corrode ferrous metals and consume the rust left behind. They had insect-like bodies covered in thick, bumpy plating, with two feathery antennae extending from their heads. Their tails ended in bony ridges that they used for balance while scurrying across stone passages. The antennae were their primary tools for feeding, as even the lightest brush could cause a weapon to tarnish and flake apart.
Though not naturally hostile, rust monsters became a great nuisance to adventurers, as their touch could ruin armor, shields, and weapons within moments. They were most often found roaming underground tunnels and caverns in search of iron, steel, adamantine, or mithral, which they could detect at a surprising distance. When they caught the scent of metal, they moved with sudden speed and focus, ignoring all else in their drive to feed. They cared little whether the rust they consumed came from a fallen spike, a forgotten tool, or the prized blade of a warrior. Adventurers often escaped by tossing scrap metal behind them, distracting the creature long enough to flee.
Rust monsters typically wandered alone or in small clusters, rarely gathering in large numbers. Their constant search for food brought them into contact with many other inhabitants of the Underdark, who regarded them as largely harmless unless provoked. Since they showed no interest in flesh and posed little danger beyond their corrosion, some denizens of the deep tolerated their presence, and in rare cases, rust monsters even became companions when well fed and cared for.