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  • Churning, Squirming Vermin – Sewer Rat Encounter for D&D 5e
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Plague rodent encounter for parties of 3rd-level or higher, with stat blocks for use in D&D 5e.

Threats of violence ring out through the normally peaceful city streets. They pour from drains and sewer openings, telling of an impending invasion that will flood the surface world and see its people subjugated. These warnings, which sound out day and night and from different areas of the city, have increased in frequency and fervency as the fated day approaches. And they are starting to annoy the townsfolk.

What began as confusion over the garbled yelling has developed into mild irritation, with several citizens even lodging complaints with the city guard. These complaints have reached a quantity that obligates the guard to investigate, as per their protocols. They have searched the most easily accessible areas of the sewers and found nothing but the expected grime and vermin, with no signs of anything larger than a rat residing in the tunnels at all. Yet the threats continue, pressuring the guards to venture further into the sewers.

The guards have been dissuaded by the prospect of wading through the sewers’ muck and instead agreed upon the more convenient option of having adventurers do it for them. Several noticeboards now display offers of a reward for the cessation of the disturbances and the apprehension of whoever is behind them. They note that the individual or group responsible should be brought in alive, as the crime does not warrant a deadly response, and feature almost indecipherably fine print directing adventurers to the barracks for more information.

Mapping the disturbances. Should the party accept the job and seek out the guard barracks, they are given access to the scant information that the guards have gathered. This includes the few areas that they investigated, which are located near the residences of those who issued complaints, the names of whom they can also provide. The guards can mark these areas on a map and give approximate directions of the sewers’ major pipes.

A character that views a map of the disturbances and that succeeds on a DC 10 Intelligence (Investigation) check recognizes that the complaints all originated from areas of social activity, such as markets and parks. Should a character read the complaint reports and succeed on a DC 13 Intelligence (Investigation) check, they notice a pattern: the times listed on the reports show that the yelling followed a path running a lap through the city. This suggests that it is the work of a single individual moving quickly through the pipes, rather than an organized group acting simultaneously.

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The Sewer Pipes

You push forward through the rusted pipes, the sounds of straining metal and chittering claws echoing from every direction. The floor of the pipes is caked in deep grime that your boots sink into with every step, squelching and forcing putrid liquid from the solids. The darkness is a blessing, as what few beams of light that filter in from above only expose the sickening shades and textures beneath your feet, the sight worsening the smell that forces its way through your lungs and stomach with every breath. And though it repels you with every glance, it seems to have attracted its own inhabitants, as fungi grow around pools of noxious green fluids and beady eyes glitter from beneath the darkened mounds.

The sewer pipes beneath the city are a convoluted network of old and new tunnels that wind beneath the cobbled streets. They have been expanded as the city grew, with new pipes being grafted onto existing ones in a confusing sprawl with no thought for navigation. Adding to this is their insufficient upkeep which has left the pipes clogged with grime, sludge, and other unmentionable detritus. Small brick chambers can be found dotted amongst the pipes, most often serving as access points, though they are in similar states of unclean disrepair. This putrid environment fosters all manner of vermin and sickness. But the worst of it is in the oldest of the pipes, where the collected muck has given rise to a uniquely strange poison.

It is in these, the deepest of the sewer pipes, that a colony of rats has amassed and flourished. It is here that Simon Baxter fled and found himself amongst the vermin. And it is in the sewer’s oldest filth that these residents became infected with the festering poison.

Map & Asset Downloads

The city’s sewers and Simon’s hideout have been created to use with our own ‘Sewer Pipes’ battle map, with the colony’s poison designed to fit with its green, acidic variant. If you’d prefer another map or even to construct your own, we have a wide variety of assets to do so…

Simon Baxter

(CE male human plague wererat)

The man sniffs at the air, his nose twitching and beady eyes flashing erratically. Grime soils every inch of his form, sticking his roughly cut hair in place and gathering beneath long, sharpened fingernails. It stains most of his clothes brown, almost hiding the amateur repairs, frayed edges, and faded purple cloth of his coat, and sticking the many frills of his once-white shirt into sodden clumps. 

A lone man lives in the depths of the city sewers, hiding and scavenging to survive. He makes no contact with the world above and chooses to instead surround himself with the rats that live in the pipes, with whom he has formed a close bond. But he has not forgotten the surface world and its transgressions.

This sewer-dweller, once known as Simon Baxter, was an affluent noble and frequenter of the city’s high society. After one such party, Simon was drunkenly stumbling home when he was accosted and infected with lycanthropy. He hid his condition, terrified by the thought that it might cost him his position in the aristocracy. It was during a later soiree that Simon’s worst fear came to be. He arrived at a masquerade ball without the titular costume, the letter informing him of the party’s details having been lost without ever reaching him. A single chuckle from the crowd sent Simon fleeing from the indignity and sequestering himself in the sewers where he believed he now belonged. Simon Baxter has not been seen since.

Simon’s embarrassment has fermented into vengeful wrath in his isolation. His lycanthropy has similarly developed, his rodent side becoming suffused with the strange poison that seeps from the sewers’ muck. He has rallied an army of sickly vermin to his side and now prepares for his triumphant return. He will subject the surface world to the same ignominy that he once felt. They will be driven before him and made to dance to the new social hierarchy, forced to serve in the empire that rises from the sewers, with Simon as its resplendent leader.

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Simon’s Sewer Colony

The sprawling pipes of the city sewers chitter with echoed footsteps. Swarms of tiny feet scamper through the darkness and dirt, their positions hidden by labyrinthine pathways as they draw closer to their prey. But what arrives are not simple sewer rats. They fester with noxious poisons, swarm in unnatural coordination, and march under the command of a single master.

Plague Rat

CR 1/8

A rat skitters forth from the shadows, twitching and sniffing. Its fur is matted and slick and parts around bulbous growths that pulse a noxious green fluid through its veins.

The rats of the city sewers are infected with a peculiar sickness that causes their bodies to produce poison. These plague rats, as witnesses have taken to calling them, are quick to use this poison against anyone or anything that so much as steps near them, scratching and biting at their targets. They also have a disgusting habit of bursting upon death, their body giving way to the poison and releasing it in a violent splatter.

Swarm of Plague Rats

CR 1/2

Rats writhe and tumble over each other in a swarming puddle of vermin, each one bubbling with green pustules and veins. Whatever sickness poisons them seems to have driven them into a frenzy. They scratch and claw through the environment, moving as if they are a single, frenetic creature.

Perhaps the most curious symptom of the rats’ illness is the way it links the colony in a network akin to a hivemind. The rats do not communicate directly but are driven by a shared instinct that causes them to often attack in swarms of plague rats. These swarms turn the normally insignificant rats into a threat capable of overwhelming larger targets, particularly as individual members of the swarm are killed and explode in a spray of toxin, poisoning their enemies without affecting the remaining rats.

Dire Plague Rat

CR 1/2

A creature claws forward, sharp talons dragging a shivering mass of fur. It is a rat, or perhaps was once a rat, as it is now the size of a man and has mutated with bubbling growths across its body. Green fluid filters from these growths, highlighting the rat’s veins and dripping from its skin.

In the depths of sewers, sustained by diets of eclectic refuse and the infected bodies of their own kin, some rats can grow to monstrous proportions. These dire plague rats are an exaggeration of the other vermin, from their appearance to their abilities. They are stronger and faster and the poison in their body leaks out at the slightest contact and spits from their mouths in globs. These dire rats are more aggressive than their regular-sized brethren and tend to hunt far larger prey, dragging their victims into the tunnels to feed the colony.

Plague Rat King

CR 1

A chorus of chittering heralds the arrival of a mound of infected, pustule-ridden rodents. A central rat of monstrous size is coated in its smaller kin, the vermin all tangled together by their tails and matted fur. Each one fights and scrambles for itself, causing the combined mass to roil and tumble, yet it maintains its form as it shambles forward.

Rats scrambling over each other in the grime-ridden confines of sewer pipes can cause them to become tangled together. And when this occurs to vermin bearing the sewer’s strange, mind-linking poison, driven on by the command of a plague wererat, they may bind together in the form of a plague rat king

The rat king is a chaotic amalgamation of plague rats and dire plague rats. It combines their abilities, its many minds sharing barely enough instinct to function as a single creature. This adds to its ferocity in battle as it attacks wildly and with little regard for the lives of the individual rats that comprise it. When surrounded, the rat king may even sacrifice some of its rats in a powerful slam that spreads poison in a radius around where it strikes.

Plague Wererat

CR 3

They move with a hunch, scrabbling as if undecided between being bipedal or quadrupedal. In their scarce flashes of visibility, you spot a pair of beady eyes and wirey grey hair, which seems to spread across more and more of the individual’s body in each moment you can see it. This layer of fur is accompanied by noxious green that filters through its veins, collects in pustules on its skin, and drips from its teeth and claws.

The strange poison that suffuses the sewer rats is not limited to those who are purely rodents. Wererats, such as Simon Baxter, may also be transformed into plague wererats. These mutations grant the individual abilities similar to the other infected vermin, though their human side allows them to wield them to greater effect. Those who embrace the change may also learn to deepen their connection to the rodents’ hivemind through the poison that plagues them. And by exercising their greater intelligence, they may even gain a position of dominance over the rats.

Balancing the Sewer Colony

The encounter’s only objective is the apprehension of Simon Baxter; without him, the rats pose little danger to the city. This means that the challenge rating of the central fight should begin with a lone plague wererat, reinforced by a selection of other rodents to achieve the desired CR.

It is important to keep the encounter’s tone in mind when choosing creatures to include. The story is designed to be humorous, with Simon’s threats being a futile annoyance rather than a legitimate attack. Maintaining this tone means that the encounter with Simon should lean towards being easier rather than deadly. If he and his rats are too dangerous for the party, his threats against the surface world gain merit, which may undercut the point of presenting Simon as feeble and petty. This is fine if it is the way you wish for the encounter to go, it simply means that it is important to decide which tone you are aiming for and to factor that into the combat’s difficulty.

The sewers’ labyrinthine pipes are also perfect for including numerous smaller encounters on the way to finding Simon. This allows you to include a greater number and variety of creatures without needing to stuff them all into a single fight. You can also reinforce the colony with regular rats, giant rats, and swarms of rats, or even branch out and include other appropriate creatures, such as oozes.

Here are some examples of creatures that you may use to achieve different CR values for the encounter with Simon:

CR 3 1/2: 1 plague wererat, 4 plague rats

CR 4: 1 plague wererat, 1 swarm of plague rats or 1 dire plague rat, 4 plague rats

CR 5: 1 plague wererat, 1 plague rat king, 1 swarm of plague rats or 1 dire plague rat, 4 plague rats

CR 7: 1 plague wererat, 2 plague rat kings, 2 swarms of plague rats, 1 dire plague rat, 4 plague rats

Looking for some custom ooze creatures to accompany the sewer rats? We have a trilogy of ooze encounters that have a total of 15 unique oozes, all with their own strange abilities…

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About the author

Troy McConnell

Part-time DM and author of 2-Minute Tabletop's encounters, map lore, and characters. Basically, I write about all the campaign ideas that I don't have time to run. All with the assistance of my feline familiar, Wink.

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