AC 15 (natural armor)
HP 27 (5d8 + 5; bloodied 13)
Speed 40 ft.
Proficiency +2; Maneuver DC 11
Senses darkvision 60 ft., passive Perception 11
Languages —
Metal Detection. The rust monster can smell metal within 30 feet.
Rust Metal. A nonmagical weapon made of metal that hits the rust monster corrodes after dealing damage, taking a permanent -1 penalty to damage rolls per hit. If this penalty reaches -5, the weapon is destroyed. Metal nonmagical ammunition is destroyed after dealing damage.
ACTIONS
Antennae. The rust monster corrodes a nonmagic metal object within 5 feet. It can destroy up to a 1-foot-square portion of an unattended object. If the object is worn or carried, the object’s owner makes a DC 11 Dexterity saving throw , avoiding the rust monster’s antennae on a success.
Metal shields or armor the rust monster touches with its antennae corrode, taking a permanent -1 penalty to its AC protection per hit. If the penalty reduces the armor’s AC protection to 10, the armor is destroyed. If a metal weapon is touched, it is subject to the Rust Metal trait.
Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 1) piercing damage.
REACTIONS
Defensive Bite. When the rust monster is hit by a melee attack made by a creature it can see within 5 feet, it bites the attacker.
Combat
On its turn, the rust monster uses its antennae if there is any metal nearby. Otherwise, it runs away. It also runs away if bloodied . It uses its reaction to bite if attacked.
Legends and Lore
With an Arcana or Nature check, characters can learn the following:
DC 10 Rust monsters are insect-like creatures that live underground and eat metal.
DC 15 A rust monster’s feathery antennae corrode any metal they touch.
DC 20 Rust monsters can corrode any type of metal, including mithral and adamantine, but not gold.
Rust monsters, also known as ore-mites, smith-banes, and kettle-eaters, are subterranean pests that feed on metal. They eat naturally occurring metal ore, and also magically corrode forged metal to feed on the resulting rust. Resembling mastiff-sized insectile armadillos, rust monsters are covered by thick, chitinous armor and have two feathery antennae that twitch expressively.
Instinctive Opportunists. Rust monsters' keen sense of smell attracts them to metal over long distances. They’ll ruin cutlery and cooking pots as often as armor, shields, and weapons. Generally docile, they are nonetheless persistent and will shadow adventurers with metal armor and weapons, awaiting opportunities to devour desirable equipment while the owners rest or are otherwise not paying attention.
Readily Tamed. Rust monsters can corrode and eat most metals, including copper, silver, mithral, and adamantine, but not gold. They prefer iron and steel, favoring these over other metals when available. They can be appeased or distracted with offerings of iron and, if well treated and well fed, become friendly. Tame rust monsters can be found living among other Underland creatures, acting as watch beasts or trained for battle against metal-armored foes.
1–3 Hungry and aggressive; charges the nearest source of metal
4 Peckish; approaches cautiously but flees if it is met with hostility
5 Happily devouring a bulky metal object such as a metal door, giant cauldron, or suit of full plate mail; ignores other metal sources for the next minute
6 Eating treasure, such as silver coins
1 Mounds of rust flakes
2 A wooden pickaxe handle
3 A wooden door lying on the ground, its handle and hinges gone
4 Scattered arrows missing their arrowheads
Rust monsters infest caverns and mines.
CR 0–2 1 or 2 rust monsters
Treasure collapsed treasure chest (missing hinges, lock, and metal studs), empty except for a carnelian gemstone (50 gp) that might have been part of a ring