Rust Monster

Medium monstrosity, unaligned

Armor Class: 14; natural armor
Hit Points: 27; 5d8 + 5
Speed: 40 ft.

STR +1
DEX +1
CON +1
INT -4
WIS +1
CHA -2

Senses: darkvision 60 ft., passive Perception 11

Challenge 1/2 100 XP · Proficiency +2

Traits

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. Non magical 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, it 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 to run it

The rust monster does almost no damage and is one of the most hated creatures in the game, because it eats equipment. Iron Scent means it makes straight for the character in plate armour, and Antennae destroys armour, shields and weapons on a failed DC 11 Dexterity save.

It is not malicious and it is not smart. It is hungry. Play it as an animal that wants the metal and has no interest in killing anyone, which makes driving it off with a thrown sword a legitimate and satisfying solution.

Building the encounter

One for a party of four at level 2 or 3, and think hard before using two. The threat is not lethality, it is loss, and loss stacks badly.

Be honest with yourself about your table before placing one. A group that enjoys resource pressure will love it. A group whose fighter has been saving for that plate armour for six sessions will not, and there is no wrong answer about which group yours is.

What to pair it with

Something that forces the party to stay and fight rather than back away, since backing away is the correct answer to a rust monster. Even a couple of skeletons blocking the exit changes the calculation.

A rust monster in a room with a hoard of metal is the better version: it gives the party something to bargain with, and throwing coins to distract it is more fun than fighting it.

Reskins

A creature that consumes a resource rather than hit points. Reflavour as something that eats magic, leather, wood, or spell slots. The mechanic is loss, and any loss the party feels will work.

If your table dislikes equipment damage, reskinning to eat something less permanent, like rations or torches, keeps the tension without the resentment.

Commonly played wrong

Rust Metal triggers when a nonmagical metal weapon HITS the rust monster, and it is cumulative. Players who keep attacking with the same sword are destroying it themselves, which is worth saying out loud before the third hit rather than after.

Magical weapons are entirely unaffected, and so is anything not made of ferrous metal. A quarterstaff, a sling or an unarmed strike works perfectly, and a party that adapts should be rewarded rather than pursued.

Compared to others at this CR

At CR 1/2 the rust monster is nothing like the orc or the hobgoblin. It will not kill anyone. It is on the list because of what it costs, not what it threatens.

Challenge rating measures danger to hit points and completely fails to capture this creature. Treat its CR as advice about its durability and nothing else.

Remix in the Monster Creator Add to an encounter Check this rating