Copyright 2008 Wizards of the Coast

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Rust Monsters are a D&D original, inspired by some cheap plastic “dinosaur” figures Gygax had lying around and intended to use as miniatures. In 4e they first appear in the Monster Manual 2 and later in the Monster Vault, so we’ll be peaking ahead a bit.

The Lore

Rust monsters came about naturally, even though plenty of people swear they were created by a spiteful god who thought mortals had too much shiny stuff. Everyone hates these things.

Rust monsters feed on metal, by secreting powerful enzymes which dissolve it and slurping the slurry. Normally they target ore veins and leech the metal out of them, but items made of processed metal draw them like blood in the water draws sharks. Since food is usually scarce, rust monsters tend to have only one or two young per brood, but when they find a plentiful source of metal like a lode or a civilized settlement, their broods increase in size to make use of that bounty.

It’s not just adventurers who curse these creatures! They get into civilized settlements and mines and eat away at all the metal tools, fittings, and support beams if you let them. Some dwarf and duergar settlements develop complex wards to keep them at bay, but their effectiveness is limited - some rust monsters have a knack for finding holes in them.

The bright side of all of this is that rust monsters can be a source of residuum, that precious substance which is used to fuel magic rituals and enchant items. You see, most of them can’t digest magic, so any magic impregnating the metal they eat ends up stored in their bodies as clumps of residuum. This is enough incentive for some people to go out hunting rust monsters, despite the risks to life, limb, and loot.

Some rust monsters are able to digest residuum, which causes them to grow much larger and to crave magic as much as they do metal. These enhanced monsters are known as Dweomer Eaters.

A Note on Residuum.

Residuum gets introduced in the PHB, so I haven’t talked about it too much here. It’s basically concentrated, crystalized magic: I imagine it as rainbow-colored sparkly dust. The rules measure it by its monetary value (for example, “100gp worth of residuum”), so I imagine it’s extremely valuable for its weight and only found in tiny quantities.

When you disenchant a magic item using a ritual from the PHB, you get a fifth of its monetary value back as residuum, which ensures you can always get some benefit even from items no one in the party wants. The substance can be used in place of the traditional components for any ritual, no matter what its power source is. And of course, Enchant Item is a ritual, so you can use it to make other magic items as well.

The Numbers

Most rust monsters are Medium Natural Beasts, though Dweomer Eaters are Large. They have low-light vision.

In earlier editions rust monsters could destroy metal weapons, armor and items instantaneously, but this has become a more gradual process in 4e. Even so the MM2 has this big box about being careful with rust monsters, because players loathe losing their magic loot even more than they fear losing their PCs.

Usually, at most one of the party’s items risks total destruction at the antennae of a rust monster per fight, though that danger multiplies if you’re fighting more than one and the penalties it inflicts can make fighting the other monsters in the encounter more difficult than they would otherwise be.

Rust Monster (Both)

Basic rust monsters are Level 6 Skirmishers with 66 HP. Their speed is 8, so you can’t easily run from them once they catch the scent of your expensive gear.

A rust monster’s bite does physical damage and rusts any heavy armor the target might be wearing. Rusting armor has a -1 penalty to AC, and multiple bites increase this to a maximum of -5. This only lasts until the end of the encounter.

Rusting Defense means attacking the monster with a metal weapon also rusts it, which inflicts a -1 penalty to attack rolls and is cumulative to a maximum of -5.

Rusted weapons and armor return to normal after the end of the encounter, so this is not quite as bad as it looks. However, the monster can target a rusting weapon or suit of armor with its Devour Metal ability (melee 1 vs. Reflex), which destroys the item in question on a hit!

If the destroyed item was magical, you’ll be able to retrieve residuum from the monster’s belly equal to the full value of the item, which doesn’t necessarily ensure you’ll get your item back but is still a bit of a consolation prize.

The main difference between books, aside from the usual damage update, is in how Devour Metal works. The MM2 version only affects magic items of 10th level or less (or nonmagic items), and is an encounter power. The MV version is actually more dangerous: it has no level limit and recharges when the monster misses with it, which means it can keep trying until it manages to destroy something.

Fighting a rust monster still produces that funny inversion effect where the squishies wearing cloth and bearing wooden weapons move to the front line and the fighters cower in the back, but now the fighters aren’t instantly doomed the moment the monster slips by the squishies.

Gluttonous Rust Monster (MV)

This Level 8 Brute has 110 HP, Speed 6, and all the abilities of the basic rust monster above.

In addition, it has a Gluttonous Bite encounter power (melee 1 vs. AC) which does more damage than the basic bite and rusts the target’s weapon and armor at the same time. On a miss, it does half damage.

Young Rust Monster Swarm (Both)

This is what you get when a couple of rust monsters nests close to a large source of metal. The swarm is a Level 9 Soldier with 92 HP. As a swarm, it takes half damage from single-target attacks and 10 extra damage from area effects. It can also squeeze through any opening wide enough to allow a single rustling through.

The swarm has Rusting Defense and its Swarm of Bites also rusts armor. Its Swarm Attack aura 1 deals 5 damage to enemies caught inside and, if they have a rusting item on them, slows them for a turn.

Dweomer Eater (MM2)

Dweomer Eaters are Large, and Level 11 Skirmishers with 110 HP. They work pretty much like standard rust monsters, with bigger numbers. There is one important difference, though: their rusting abilities target not metal, but magic.

This means that any magic armor or weapon used by the party, no matter what material it’s made from, is at risk when fighting a dweomer eater. Mundane items are fine, but that is scant consolation for a level 11 party typically clad in +2 or +3 armor and weapons. Yes, if you hit them with spells channeled through a magic implement, the implement rusts even if it’s made of wood or crystal.

You can still extract residuum equal in value to any items destroyed by a dweomer eater from its corpse, at least.

Sample Encounters and Final Impressions

The encounters we have in the MM2 place rust monsters alongside creatures which don’t usually employ metal weapons, such as lizardfolk that fight unarmed or with magic, and certain faeries. I don’t think you can train a rust monster not to eat your metal weapons, so people who rely on steel won’t be using them as pets.

I like rust monsters! The original versions terrified me as a player, but the kinder implementation here feels just right. A single mistake won’t cost you your hard-won loot, and the GM gets to have a bit more fun by prolonging the dread.

PCs with Enchant Item can make magic items with a maximum level equal to their own, so the residuum refund from a slain rust monster makes the loss of such an item temporary. You usually find items of a higher level than that as treasure, though. The Essentials rules also introduce the concept of item rarity, which makes certain items impossible to re-create via ritual. Both of these things mean some losses are indeed permanent or at least very difficult to replace.