AC 16 (natural armor)
HP 126 (12d10 + 60; bloodied 63)
Speed 45 ft
Proficiency +3; Maneuver DC 16
Saving Throws Dex +4, Wis +4
Damage Resistances acid, cold, fire, lightning, poison, psychic, thunder
Condition Immunities charmed, frightened
Senses darkvision 60 ft., passive Perception 11
Languages —
Draining Aura. When a living creature starts its turn within 20 feet of the rust monster or enters the area for the first time on a turn, it must succeed on a DC 16 Constitution saving throw or take 3 (1d6) force damage as its blood is sucked from its pores by the rust monster’s feathery antennae. The creature’s hit point maximum is reduced by an amount equal to the force damage taken, and it has disadvantage on attack rolls and ability checks until the start of its next turn. The reduction lasts until the creature finishes a long rest or is targeted by a greater restoration spell. The target dies if its hit point maximum is reduced to 0.
Metal Detection. The rust monster can smell metal within 60 feet.
Rust Metal. A 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. A creature can spend 1 minute polishing or repairing a magic weapon, removing this penalty. Metal ammunition is destroyed after dealing damage.
ACTIONS
Multiattack. The rust monster attacks once with its antennae and twice with its bite.
Antennae. The rust monster uses its antennae to corrode a metal object within 20 feet. It can destroy up to a 1-footsquare portion of an unattended object. If the object is worn or carried, the object’s owner makes a DC 16 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. A creature can spend 1 minute polishing or repairing magic armor, removing this penalty. If the rust monster touches a metal weapon, the weapon is subject to Rust Metal.
Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 18 (3d8 + 5) piercing damage. If the target is a Large or smaller creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained and the rust monster can’t bite another target.
Combat
The rust monster attacks the creature carrying the most metal; once all metal is gone, it targets the closest creature. It flees if it is bloodied and it detects no iron or steel items within 60 feet.
Veteran dungeon delvers often regard rust monsters as pests but rarely as threats in their own right. Some rust monsters, though, have been transformed into fearsome predators by magic run amok. With coiled antennae that extend to astonishing distances, these rust monsters seek not only to consume an adventurer’s weapons, but also the iron in their blood.
Rust monsters gravitate to strongholds and other places where worked iron is plentiful, but they can also be found in mines.
CR 5–10 mutant rust monster
CR 11–16 2 mutant rust monsters ; supermutant rust monster
Treasure golden sword pommel decorated with gems but missing its blade (2,500 gp