Rust Monster 5e: What It Ruins, What It Can't Touch, and the 2024 Rules

A rust monster is a CR 1/2 Medium Monstrosity with AC 14, 27 hit points (33 in 2024) and a weak +3 bite. It barely threatens anyone's hit points. It threatens their gear: its antennae corrode nonmagical metal armor and weapons, a little at a time, until they fall apart. In the 2014 rules any nonmagical metal weapon that hits it also corrodes. The 2024 rust monster drops that trait, uses its antennae twice a turn plus once as a reaction when hit, and its damage can be undone with the Mending cantrip.

Table of Contents

• •Rust Monster Stat Block (2014)
• •The Rust Monster in the 2024 Rules
• •What a Rust Monster Can and Can't Ruin
• •How Rust Monsters Fight
• •Running a Rust Monster Encounter
• •How Players Beat Rust Monsters
• •Rust Monster Encounter Ideas
• •Frequently Asked Questions

Rust Monster Stat Block (2014)

The rust monster from SRD 5.1:

The 2014 version has no Multiattack. Each turn it either bites or uses Antennae, never both. Its damage to equipment comes from two places:

• •Antennae (its action). It touches a nonmagical ferrous (iron or steel) object it can see within 5 feet. If someone is wearing or carrying it, that creature makes a DC 11 Dexterity save to avoid the touch. Armor or a shield that's touched takes a permanent, cumulative −1 to the AC it gives. A held weapon takes −1 to damage rolls. An unattended object loses a 1-foot cube outright.
• •Rust Metal (passive). Any nonmagical metal weapon that hits the rust monster takes a permanent, cumulative −1 to damage rolls after dealing its damage. No save. Nonmagical metal ammunition that hits it is destroyed after dealing damage.

The Rust Monster in the 2024 Rules

SRD 5.2.1 keeps CR 1/2 (100 XP), AC 14, the 40-foot speed and the +3 bite, and moves all the rusting into its actions.

The 2024 version is harsher in combat and kinder afterward. It can make three Antennae attempts in a round (two from Multiattack, one as its reaction), and a weapon penalty now hits attack rolls instead of damage. But Rust Metal is gone, so a weapon that hits it is safe unless the reaction targets it. And one casting of Mending removes the penalty from a damaged item.

What a Rust Monster Can and Can't Ruin

The most common table argument about rust monsters is which items count. Here's what the stat blocks say.

How long gear lasts: chain mail (AC 16) is destroyed after 6 failed saves, plate (AC 18) after 8, a breastplate (AC 14) after 4. A 2014 shield (+2) survives two. Against a fighter in plate with Dexterity +0, each Antennae attempt succeeds half the time. A 2024 rust monster that uses all three attempts on that fighter takes about 1.5 points of AC per round.

Metal parts of nonmetal items (studs on studded leather, a spear's head) are the DM's call. Decide before the fight and say so.

How Rust Monsters Fight

A rust monster goes for the most metal. Iron Scent pinpoints ferrous metal within 30 feet, so it finds the fighter in plate even in the dark. With 40 feet of speed it reaches the front line on turn one.

2014. Use Antennae every turn on the target with the most armor or the biggest sword. The bite (+3 vs AC 18 hits 30% of the time, for 5 damage) is a waste of an action against a knight. Every time the fighter hits back with a steel sword, the sword loses 1 damage, and the loss compounds. A longsword averaging 7.5 deals 7.5, 6.5, 5.5 and 4.5 on its first four hits: 24 damage, not quite enough to kill a 27-HP rust monster. The fifth hit takes the sword to −5 and destroys it.

2024. Bite and use Antennae twice each turn, then wait for the reaction. Any attack roll that hits the rust monster lets it use Antennae again against an item worn or carried by a creature within 5 feet of it. That creature doesn't have to be the attacker.

Two or three rust monsters in a narrow corridor, each picking a different armored character, can turn a well-equipped party into a lightly armored one in three rounds.

Running a Rust Monster Encounter

A rust monster is 100 XP. For four characters with the 2024 XP budget (SRD 5.2.1): two rust monsters are a Low encounter at level 1 (200 XP), three are Moderate (300), and four are High (400). Under the 2014 Dungeon Master's Guide guidelines (used by our encounter calculator), two rust monsters are 300 adjusted XP, Hard for four level 1 characters.

At higher levels the XP math stops describing the threat. A single rust monster can't kill a level 6 party, but it can cost the fighter 3 points of AC for the rest of the dungeon. Use one as a complication: a rust monster wandering into a fight against hobgoblins, or a pair guarding a forge, and tell players up front if lost gear can be replaced in town.

How Players Beat Rust Monsters

• •Switch weapons. Clubs, quarterstaffs, greatclubs, unarmed strikes and spells don't care about rust. A 2014 rust monster has only 27 hit points and AC 14.
• •Use magic gear. Magic weapons and armor are immune. If someone has the Magic Weapon spell (a paladin or wizard, for example), a steel sword becomes a magic weapon for the next hour.
• •Shoot it from range in 2024. Reflexive Antennae only reaches creatures within 5 feet of the rust monster. An archer 30 feet away loses nothing. In 2014 every metal-tipped arrow that hits is destroyed, which is cheap.
• •Take the shield off. Doffing a shield takes an action. Body armor takes minutes, so the fighter in plate should stay back while others handle it.
• •Feed it. Its whole body is built around finding and destroying iron. As a DM call, a sack of iron spikes thrown away from the party can pull its attention for a round.
• •Keep Mending handy (2024). One casting removes the penalty from a damaged item. In 2014 the penalty is permanent and the text doesn't mention Mending, so allowing it is a house rule.

Rust Monster Encounter Ideas

The armory. A garrison has 40 sets of chain mail and no idea why they're rusting overnight. Someone is letting a rust monster into the storeroom through a drainage grate every night. The culprit is a quartermaster paid by bandits who plan to hit the fort when its soldiers have nothing to wear.

The pilgrim's corridor. A shrine at the end of a long tunnel is guarded by three rust monsters kept fed by the shrine's keepers. Pilgrims leave their metal at the entrance. The party can follow the custom, fight, or try to sneak through with a pack mule loaded with iron as a distraction.

The smith's request. A blacksmith offers 100 gp for a live rust monster in a wooden cage, to test a new alloy. Catching one alive without losing any gear is the challenge. A net, a wooden cage and a bag of iron filings will do it if the party plans.

Frequently Asked Questions

What CR is a rust monster in 5e?

A rust monster is CR 1/2 and worth 100 XP in both the 2014 and 2024 rules. It has AC 14 and 27 hit points (33 in 2024).

Can a rust monster destroy magic items?

No. Antennae, Rust Metal (2014) and Destroy Metal (2024) all affect only nonmagical metal. A +1 sword, Adamantine Armor or Mithral Armor is safe.

Can Mending fix armor damaged by a rust monster?

In the 2024 rules, yes: casting Mending on the armor or weapon removes the penalty. The 2014 stat block calls the penalty permanent and doesn't mention Mending. Neither edition lets Mending bring back an item that was destroyed.

What does a rust monster do to weapons?

In 2014, a nonmagical metal weapon that hits it, or that its Antennae touches, takes a permanent −1 to damage rolls, and it's destroyed at −5. Metal ammunition that hits it is destroyed. In 2024, only Antennae affects weapons: −1 to attack rolls per failed DC 11 Dexterity save, destroyed at −5.

Does a rust monster affect leather armor?

No. Its abilities target metal objects only, so leather, padded and hide armor, wooden shields and wooden weapons are unaffected. Loreify can record your session and keep track of the loot and losses, so nobody forgets whose shield got eaten: see the D&D note-taking app.

Stat blocks: System Reference Document 5.1 (2014) and 5.2.1 (2024), Wizards of the Coast, CC-BY-4.0. Tactics, encounter math and ideas are Loreify's own.