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.
Actions
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.
If the object touched is either metal armor or a metal shield being worn or carried, its takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. If the object touched is a held metal weapon, it rusts as described in the Rust Metal trait.
How rust monsters fight
A rust monster is a CR 1/2 insect-like monstrosity with 27 hit points and a weak bite (1d8 + 1). It doesn’t want to kill anyone. It wants to eat metal, and it can smell ferrous metal within 30 feet.
Its Antennae touch a nonmagical ferrous object within 5 feet: an unattended object loses a 1-foot cube, and a creature can make a DC 11 Dexterity save to keep its gear out of reach. Metal armor or a shield that’s touched takes a permanent −1 to the AC it gives; armor at AC 10 or a shield at +0 is destroyed. And any nonmagical metal weapon that hits the rust monster corrodes: a permanent, cumulative −1 to damage, destroyed at −5. Metal ammunition that hits it is destroyed outright.
Running a rust monster at the table
Go for the plate armor. It’s drawn to metal, and the fighter in full plate is the richest meal in the room.
Let it be a problem, not a killer. Its bite rarely drops anyone. The threat is losing gear, so it’s a good encounter for players to solve rather than just fight.
Let players distract it. It’d rather eat than fight. A thrown bag of iron nails or a spare shield buys time.
Warn the party. Losing a favorite weapon feels unfair without some foreshadowing: tracks, a pile of corroded armor, rust stains in the corridor.
Common questions
Can a rust monster destroy magic items?
No. Rust Metal only affects nonmagical weapons, and Antennae only nonmagical ferrous metal. Magic weapons and magic armor, including mithral and adamantine armor, are safe.
How do you fight a rust monster without losing your weapon?
Use a magic weapon, a weapon that isn’t metal (a club, quarterstaff or wooden weapon), unarmed strikes, or spells. Arrows with metal heads are usually treated as metal ammunition and destroyed on a hit.
Does the rust monster’s touch need an attack roll?
No. Antennae is an action that corrodes an object it can see within 5 feet. If the object is worn or carried, the creature gets a DC 11 Dexterity save to avoid the touch.
Can you repair rusted armor?
The rules call the penalty permanent and give no way to repair it. Mending only fixes a single break or tear, so whether it helps is the DM’s call. Replacing the gear is the usual answer.
How much XP is a rust monster?
100 XP at CR 1/2. One is an Easy fight for four 1st-level characters by the Dungeon Master’s Guide’s encounter math, until you count the cost of new armor.
Did the rust monster change in 2024?
Yes. The 2024 rust monster has 33 hit points, makes one bite and two Antennae uses each turn, can use Antennae as a reaction when it’s hit, and its corrosion lowers a weapon’s attack rolls instead of its damage.
CR 2. Another insectile monstrosity, which burrows, bites, grabs and sprays acid.
How many rust monsters for a fight?
A rust monster is worth 100 XP. Using the 2024 encounter budgets (SRD 5.2), this is how many rust monsters fit each difficulty for a party of four characters of the same level. A dash means even one would be over the budget.
Party level
Low
Moderate
High
1st
2
3
4
2nd
4
6
8
3rd
6
9
12+
4th
10
12+
12+
Budgets add up every monster’s XP, so you can swap some for other creatures of the same total. Try other party sizes and mixes in the encounter calculator.
How the rust monster compares at CR 1/2
Against the 32 other challenge 1/2 monsters in the SRD 5.1, the rust monster has more hit points than 29 of them, which makes it one of the toughest at its CR. See every CR 1/2 monster.
Statistic
Rust Monster
CR 1/2 average
Armor Class
14
12.4
Hit points
27
20
Best attack bonus
+3
+4
Rust Monster 2024 stat block
The 2024 Monster Manual revised the rust monster, and the SRD 5.2 carries the new stat block. What changed:
Hit points: 27 → 33.
New in its stat block: Multiattack, Destroy Metal and Reflexive Antennae.
Gone from it: Rust Metal.
Armor Class
14
Hit Points
33 (6d8 + 6)
Speed
40 ft.
Challenge
1/2 (100 XP)
STR
DEX
CON
INT
WIS
CHA
13 (+1)
12 (+1)
13 (+1)
2 (-4)
13 (+1)
6 (-2)
Senses
darkvision 60 ft., passive Perception 11
Languages
None
Traits
Iron Scent. The rust monster can pinpoint the location of ferrous metal within 30 feet of itself.
Actions
Multiattack. The rust monster makes one Bite attack and uses Antennae twice.
Antennae. The rust monster targets one nonmagical metal object—armor or a weapon—worn or carried by a creature within 5 feet of itself. Dexterity Saving Throw: DC 11, the creature with the object. Failure: The object takes a −1 penalty to the AC it offers (armor) or to its attack rolls (weapon). Armor is destroyed if the penalty reduces its AC to 10, and a weapon is destroyed if its penalty reaches −5. The penalty can be removed by casting the Mending spell on the armor or weapon.
Destroy Metal. The rust monster touches a nonmagical metal object within 5 feet of itself that isn’t being worn or carried. The touch destroys a 1-foot Cube of the object.
Reactions
Reflexive Antennae. Trigger: An attack roll hits the rust monster. Response: The rust monster uses Antennae.
