5e SRD:Rust Monster
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Medium monstrosity, unaligned
Iron Scent. The rust monster can pinpoint, by scent, the location of ferrous metal within 30 feet of it.
Rust Metal. Any nonmagical weapon made of metal that hits the rust monster corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the rust monster is destroyed after dealing damage.
Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 1) piercing damage.
Antennae. The rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. If the object isn't being worn or carried, the touch destroys a 1-foot cube of it. If the object is being worn or carried by a creature, the creature can make a DC 11 Dexterity saving throw to avoid the rust monster's touch.
Most dwarves would rather face a squad of orcs than confront a single rust monster. 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.
Underground Scavengers. 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.
Subterranean Wanderers. Rust monsters are rarely found in large numbers, preferring to hunt alone or in small groups. They meander along tunnels, moving from cave to cave in their tireless search for ferrous metals to consume. Their wanderings often bring them into contact with other Underdark denizens that find them harmless or unappetizing. Thus, rust monsters may be found in close proximity to other subterranean monsters. If they are well treated and well fed, they can also become friendly companions or pets.