This insectile monster has four legs, a strange propeller-shaped protrusion at the end of its tail, and two long, feathery antennae.
Rust Monster CR 3
N Medium aberration
Init +3; Senses darkvision 60 ft., scent metals 90 ft.; Perception +12
AC 18, touch 13, flat-footed 15 (+3 Dex, +5 natural)
hp 27 (5d8+5)
Fort +2, Ref +4, Will +5
Speed 40 ft., climb 10 ft.
Melee bite +6 (1d3), antennae +6 touch (rust)
Str 10, Dex 17, Con 13, Int 2, Wis 13, Cha 8
Base Atk +3; CMB +3; CMD 16 (20 vs. trip)
Environment any underground
Organization solitary, pair, or nest (3–10)
Treasure incidental (no metal treasure)
Rust (Su) A rust monster's antennae are a primary touch attack that causes any metal object they touch to swiftly rust and corrode. The object touched takes half its maximum hp in damage and gains the broken condition—a second hit destroys the item. A rust monster never provokes attacks of opportunity by attempting to strike a weapon with its antennae. Against creatures made of metal, a rust monster's antennae deal 3d6+5 points of damage. An attended object, any magic object, or a metal creature can attempt a DC 15 Reflex save to negate this effect. The save DC is Constitution-based.
Scent Metals (Ex) This ability functions much the same as the scent ability, except that the range is 90 feet and the rust monster can only use it to sense metal objects (including creatures wearing or carrying metal objects).
Of all the terrifying beasts an explorer might encounter underground, only the rust monster targets that which the average adventurer values most: his treasure.
Typically 5 feet long and weighing almost 200 pounds, the lobster-like rust monster would be frightening enough even without the alien feeding process that gives it its name. Rust monsters consume metal objects, preferring iron and ferrous alloys like steel but devouring even mithral, adamantine, and enchanted metals with equal ease. Any metal touched by the rust monster's delicate antennae or armored hide corrodes and falls to dust within seconds, making the beast a major threat to subterranean adventurers and those dwarven miners who must defend their forges and compete for ore.
Though rust monsters have no innate tendency toward violence, their insatiable hunger leads them to charge anything they come across that bears even trace amounts of metal, and any resistance is met with unthinking savagery. It's not unheard of for rust monsters in metal-poor areas to track escaped victims for days using their scent metal ability, provided the victims retain intact metal objects. Fortunately, it's often possible to escape a rust monster's attentions by throwing it a dense metal object like a shield and running in the opposite direction. Those who frequent areas infested with rust monsters quickly learn to keep a few stone or wooden weapons close at hand.