r/DnDBehindTheScreen

Most monsters in the Manual have a predictable relationship with the living. Either they’re trying to eat you or trying not to be eaten by you.

But there is one monster in the Manual who isn’t really interested in your players.

It’s interested in their stuff.

Picture this: your Party is roaming through the Underdark, navigating by the lights of bioluminescent fungus or fiery mushrooms. Then they hear the dry clicking of chitin on stone, the shift of hard scales. They might think they’re Ankhegs or perhaps a Carrion Crawler, but it is not until tiny rust marks begin to flake off the Fighter’s sword that they realize it’s something much more terrible.

It’s a Rust Monster.

And Rust Monsters don’t take blood. They take confidence.

What makes a Rust Monster scary isn’t the attacks it can do. At CR 1/2, they’re not lethal—33 HP, a 5-damage bite. A level-2 party shouldn’t break a sweat. Physically.

The thing with physical injury in D&D, of course, is that every player knows that a Long Rest will cure just about anything. Knocked down to 1 HP by a Beholder? Hey – if you can get back to camp, you’ll wake up fresh as a daisy the next morning.

Players know injury is temporary. Gear loss? That’s a dent in the soul.

This means that an encounter with a Rust Monster will bring a different kind of fear than almost any other monster encounter, with different emotional stakes for the players. They know that the destruction of armor or a sword or other equipment will come with permanent consequence, and that’ll create a whole new level of tension.

Watch your players’ faces as you describe the metallic sizzling of their blade rusting. Chef’s kiss. Best thing ever.

So when you’re running a Rust Monster, you of course want to prioritize whoever has the most metal on them, which will almost certainly be a martial player. Not as predators. More like ambulatory garbage disposals following scent alone, and they’ll start taking apart every piece of steel they can find. This will force your other players to intercede, maybe looking for ways to draw the Rust Monsters away and reduce the chance that your Fighter will fail five DEX saves in a row.

You can amplify this with challenging terrain – narrow paths, cluttered ruins, any kind of space where retreat is difficult and potentially dangerous. How fast can you get through these monsters, and how much of your stuff are you going to lose?

You can also think about who to pair them with. They go really well with creatures that don’t use metal, of course, as well as other creatures that dissolve and eat away at things. This means putting Rust Monsters with various oozes or elementals that corrode or disrupt matter—earth elementals in rust-rich caverns, or mephits made of acidic fumes. These might create a really interesting encounter where everything is trying to not just eat the Players, but to unmake them.

There’s also some creative twists you can play with. For example, your players might come across a cavern littered with rusted blades and corroded armor, growing into a pile of rusted jagged metal. In the center, like a queen ant, sits the Rust Monster Queen, fat with trace metals and surrounded by her kin. And if, by some strange means, your Party’s McGuffin is in her nest, well, you can wish your party luck and start working on your next merchant NPC.

Give her a 10-ft Corrosion Aura: any nonmagical metal that starts its turn within range must save or take a place on the ‘oh god not my heirloom sword’ track.

Or try this – your Party has to deal with a pack of Kobolds, maybe in search of clues to a dragon’s lair. But the Kobolds are clever creatures. Before all the traps and winding warrens, they have several caged Rust Monsters. They flank the entrances, waiting for someone with some nice juicy metal to come by. Have enough of these, and your Players will be in their undershorts before the combat even begins. It’s the perfect kobold trap: cheap, vicious, and humiliating. The trifecta.

And while the Rust Monsters might not think of this, you certainly will: there are some fighters with beautifully crafted blades, armor that has been passed down for generations, a shield that carried them through countless battles.

Target them first. Just sayin’.

If you don’t want your players to stop coming to your games, of course, you can do a few things to minimize crisis. Telegraph the danger with rusted equipment near the Rust Monsters’ territory. Put a weeping fighter in a shop buying cheap plate because his gorgeous ancestral armor is now artisanal orange powder.

You could also set up alternative solutions for your players, like using scrap metal to redirect it and move it where the Party wants it to go.

If you want to test your players’ courage, throw a dragon at them. If you want to test their wits, put them in front of a Hag.

But if you want to test their resolve – give them a Rust Monster. Nothing reveals a party’s soul like the sound of a sword turning to dust.

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Last month my rogue lifted an unmarked vial off a dead cultist. Instead of reading her a description, I slid a card across the table but the card had a fat black bar covering whatever it actually did.

She carried that mystery for three sessions until the party paid to identify it and I removed the inner card sleeve to reveal the text underneath.

That "unidentified item" trick is a huge hit with my players and I haven't seen it in other card builders (I'm interested to see other implementations if anyone knows of this feature in other card builders).

It's a browser app that turns an item into a print-ready card sized to a standard Magic The Gathering card, so it sleeves up and lives in a standard mtg card binders. You can write the front and back card info in Markdown, pick a border color, hit download, cut it out and glue it to a junk MTG card. Use some black painter's tape and a perfect fit card sleeve to create the inner blackout layer, so you can hide a secret and reveal it later without redoing the card.

Cardsworth runs entirely in your browser and works offline. No account, and it can't vanish on me halfway through a campaign. Every card saves to a YAML file you own, so you can reprint or revise it forever.

Free and open source, with an optional API if you'd rather batch-print a whole hoard at once. (Note: API only works in self-host mode)

Try it live: 

How it works (supplies + step-by-step): 

Repo:  Please report any issues you find.

The terrifying thing, I think, about a Swarm of Rats isn’t that it’s rats.

It’s the sound.

A little skittering on the brick or concrete floor of the sewer tunnels. Soft at first, as one of them ventures out to see what’s out there. And then more and more – like raindrops on the roof. Incessant. Driving. But not random, like rain.

There’s a rhythm to it. A flow, as the swarm moves, rushes towards you. The clicking of hundreds of tiny nails almost overpower the eager squeaking noises coming from behind a thousand teeth as they surge towards the poor, lost giant that they intend to devour, leaving nothing but bone and memory.

Swarms as a creature type in D&D carry a certain kind of terror. Part of it is that they’re undefined. You don’t know exactly how many creatures are in a Swarm, just that they’re crawling all over you, moving around in such a way that counting them seems impossible.

Additionally, swarms can do something that few other creatures can do: it can occupy another creature’s space. It can move with that creature, biting as it goes.

That’s about as far as the Monster Manual takes it, but there’s no reason you can’t take it further.

The Swarm of Rats might not be intelligent, but they’re well-coordinated. They can perhaps use the Players themselves as a type of cover from injury – if one Player tries to hit a Swarm that’s in the same space as another Player, feel free to lovingly transfer that damage to the poor sap who’s already covered in rats.

They can slip under clothes, into open packs, and exploit wounds. Maybe they can make it harder to pick up dropped gear, or steal things from a backpack before a Player notices. They can squeeze into places your Players can’t, making them harder to track down and kill. They can climb the walls, coming at the Players from literally any direction.

It’s very easy for Players to underestimate a Swarm of Rats, but a good DM absolutely shouldn’t.

For one thing, you can use a Swarm of Rats to reward player movement. They get up on boxes and crates, set the floor on fire, or use other means of terrain control, so now it’s not just a fight, it’s a desperate attempt to change the battlefield to your advantage. The inverse of this is that you might use the Rats to move the Players, directing them towards the particular passage you want them to visit, or away from one you don’t.

Think of your Swarms like a weather pattern, rather than discrete creatures. In one part of the encounter, they’re ahead of your Party, cutting them off from safety or resources. In another, they’re behind, nipping on your Party’s heels and waiting for them to stand still for just a little too long. And when they’re too badly hurt, they split off, or surge, or flee to recover and regroup.

With a little creative thinking, misdirection, and some very stress-inducing music at the table, you’ll make your Players regret coming into the sewers today, no matter what they thought they would find down there.

Of course, it doesn’t always have to be sewers.

A small village near the city is known for its grain production. They out-harvest everyone in the region, and produce some of the best breads, flour, and other products around. They’ve become incredibly prosperous, attracting scholars from all over the continent who want to study their agricultural practices.

But once a month, everyone in the village locks themselves inside their homes, leaving a gift of grain out by their door. Usually, this grain is gone by morning.

Not this time.

This time, the Rats are still hungry.

Or perhaps the Rats gather in the forest, slinking through the underbrush and building nests under fallen logs. They bother no one, and live a reasonable, ratty life.

Recently, though, the Rats have discovered a new hole that has opened up in the ground, jagged and sulfurous. The Rats have started to gather there, listening to what lives far below. Something with a mind like broken glass and a voice that only the tiniest creatures can hear.

It’s teaching them things. New patterns. New ideas. Turning them into not just woodland creatures, but a single force that can operate at the direction of a strange and angry mind.

This is their forest now, and woe betide any luckless Adventuring Party that finds their way in.

Ultimately, it’s important to remember that a swarm more than a monster.

It’s a reminder: sometimes, in D&D as in life, danger doesn’t come in one big problem.

Sometimes it comes in a hundred, skittering small ones, all moving in the same direction.

Right at you.

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