-The Green Empire, vol 3
Some time ago I heard about “Rat Kings”, a real-world phenomena where groups of rats grow connected at the tail. They are occasionally discovered inside walls or sewer tunnels. I thought to myself for a bit about “Rat Kings”, and decided that they were an alarming way to include rats in a Dungeons and Dragons game as something other than a starting monster.
Rats are one of the more tame monsters in Dungeons and Dragons, and generally pose no threat to most adventurers. To change that, I wrote this low-level sidequest idea, which focuses not only to put your party on edge but also to treat rats with a dignity I believe they are due. The trick here is numbers. This idea hinges on the party quickly becoming overwhelmed as they discover more and more rats, and retreat is not an option.
The town of Mornvale is relatively new, but has been expanding quickly in recent years. Lately though, three different buildings have had a rat problem. The general store, tavern, and an old warehouse have all been experiencing missing stores, chewed rafters, and an unidentifiable stench of death.
The party explores these three ‘mini dungeons’, each bookended by a conversation with the shop keeper or custodian, and a hint about what is going on. Put a few rats in each room, but nothing overly challenging. The point here is to lull the party into a false sense of security. In one of the three buildings, there is a hidden tunnel, or a deep crack in the stone floor. This is the source of the rats.
There’s a bit of railroading here; the party will want to explore the crack, especially if you put something shiny at the bottom of it. After they repel down though, something chews through the rope, or they find some reason to remain in the tunnel. These are general ideas; choose the one that reflects your party the best.
What the party has discovered is a tunnel leading through an old, abandoned orc temple to Yurtrus, their god of death and decay. Half of the fun of running a dungeon crawl, as a DM, is building the dungeon, so I leave that to you. Just use a lot of rats. More rats than the party can handle. You can use zombies or oozes too, and make them fit by describing them as ‘desiccated corpses covered with writhing furry bodies’. If a rat swarm seems too boring, describe them as ‘green eyed, skeletal plague rats’ and all will be forgiven. Use a few different diseases too; rats are full of nasty things.
The temple contains a supernatural Rat King as a sort of "boss monster”. Depending on which edition you're running, you could use the stat block from an Otyugh, Beholder, or Hydra as a good starting point, and flavor it up to be a massive tangle of large rats, connected at the tails, with glowing eyes. As far as attacks, anything goes: it could spew plague-ridden filth, bite and scratch, detach smaller clusters of rats when struck, or dissolve into a ghostly rat swarm and re-solidify somewhere else… the sky is the limit.
A large statue of Yurtrus (I couldn’t find a description of him, but I like the idea of a fat, rotting orc with rats crawling from his eye sockets) sits against the far wall in this chamber, casting a glowing aura of decay that seems to empower the Rat King. Maybe it gives the Rat King the ability to fly, boosts its damage, adds more hit points, grants it additional effects for attacks… whatever sounds appropriate. A cleric could spend an action chanting (Knowledge: Religion or Spell check) to disable that empowerment, or a strong character can attempt to smash the statue (Strength check or attack) to disable the effect more directly.
Destroying the statue, in any case, will get rid of the rats (they are drawn to it, or created by it) and solve the town’s problems. The party can either leave through some side tunnel, transition to the underdark, or discover a portal to another plane. Maybe this shrine is only a piece of an entire orcish ruin.
You can extend the temple by adding additional challenges, or rat-themed traps. Things like a pit trap with rats at the bottom, or a room that gets flooded by rats. You could fire or flood-based trap signaled by the squeals of panicked rodents, too.
To expand this into something larger, think about what happened before the adventurers showed up. Why wasn't the statue's power evident before the town was founded? Where did the crack/tunnel come from? What happened to the orcs who worshipped here? Did Yurtus's shrine have a relic of unholy power? How will the adventurers get rid of it?
In any case, the goal with this dungeon crawl is to make players see rats with something more than the normal boredom associated with them. If you opt to run this, let me know how it goes!