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You are at Enrique's Empanada Emporium, famous for its Terrapin Pale Ale and its stuffed baked pasties, all of which are made onsite by Enrique himself. You are meeting with Bella Teach who is giving you the lowdown.

Bella: Look, I don't know what else to say about this. The rats are swarming and acting weird. We have established routes and tunnels through the sewers that we use to get our work done, but I don't feel safe sending my associates through them until the rat issue is settled. I need you to get to the bottom of this.

She goes on to say that sometimes the rats act normally, then other times they vanish entirely for periods of time, only to reappear suddenly in great numbers, swarming through the tunnels.

She needs you to investigate and report back to her, letting her know whether or not it is safe for her to send her people back down into the sewers.

Bella gives you a map that shows all the sewer entrances around town.

She also gives you the following:

1. A hat of vermin: https://www.dndbeyond.com/magic-items/hat-of-vermin

2. A pipe of the sewers: https://www.dndbeyond.com/magic-items/pipes-of-the-sewers

3. A bag of tricks that can only summon rats: https://www.dndbeyond.com/magic-items/bag-of-tricks

She expects these magic items back when you return and give her your report.

Background: Bella Teach, aka "Teacher", is indeed the leader of the Mischief, a local thieves guild. What almost nobody knows is that most of the Mischief, and all of its inner circle, are wererats. Which makes them exceptional spies. And which also makes the rat situation feel particularly personal.

Summary: you adventure through the sewers, eventually encountering a swarm of cranium rats, and then tracking them back to the basement of a house.

You go down into the sewers. You start to encounter rats, and eventually a couple of cranium rats.

You can track them to the back of the sewer, where they disappear into the cracks of the foundation of the building above.

You must go back up to the surface to gain entrance to the building.

Sewer Encounters

1. Giant Rats
2. Swarms of Rats
3. Kudzu Room: Awakened Shrub x2, Awakened Tree, Assassin Vine
4. Cranium Rat
5. Swarm of Cranium Rats
6. Gelatinous Cube
7. Rats playing checkers (Mischief wererats in rat form)

Sewer puzzle: there is a long dead end tunnel in the sewer. Scrawled on the wall before you is "You can only go backward from here." When you turn around, there is a wall five feet behind you blocking your retreat. Solution: only by walking backwards can you go back the way you came.

Cranium Rats: The cranium swarm will approach the players and start to probe their thoughts, and commanding the players to keep them from attacking. They are patroling the sewers for an intelligent creature to lure back to Silas that won't be too much trouble, will be easy to subdue, and won't put up too much of a fight. If the players attack, or if they resist its mental probes, it will decide they are too much trouble, and will retreat through the tunnels and through the foundation of Marne's basement.

A comfortable home in Riverside, the more ritzy part of Blue Harbor. It belongs to Grandma Marne, a cherished and adored membero of the Blue Harbor community.

Challenge: getting into Grandma Marne's house. She's pretty deaf and won't hear you knocking.

If the party peers in the windows, they will see Marne at a table with two settings, drinking tea by herself. As they continue to watch, a sinister, cloaked, masked figure emerges from the back of the house and makes its way toward the front of the house where Marne is. When it is close enough for her to see it, Marne looks up and smiles. It sits down in front of the second place setting and appears to converse with Marne.

Once the players get inside, Marne is delighted to have guests. The cloaked figure, Silas, gets embarrassed and uncomfortable, insisting its presence will disturb the guests, and leave.

She has recently allowed Silas, a stranger in town, to take up lodging in the flat behind the cottage. He has had a terrible accident and wears a mask to hide his disfigurement. She pities him, but enjoys his company and likes taking care of him. He keeps to himself and occupies himself with his studies.

If asked about rats, she admits to having seen some vermin in the house. She set out traps, but hasn't caught any. She reckons they might be coming up from the cellar. She admits she hasn't been down to the cellar in ages, and the cellar door has become stuck due to disuse.

Puzzle: In the kitchen there is a hatch leading down to the cellar. The hatch cannot be opened by normal means or by magic. There is a mirror opposite the window that Marne put there to improve the lighting. When the player looks in the mirror, they see the skull of a bird sitting on top of the hatch that is not actually there. Solution: If the players find a bird skull and place it on the hatch, it will open.

Silas lives in a granny flat behind Marne's cottage. The chief thing to find here is the skull of a rook on his writing desk.

His writing desk is also covered in research. Scrolls, notes, books on the physiology and anatomy of various creatures, humanoid and and otherwise.

In the basement the players find Silas, experimenting on some rats, about halfway through creating a cranium rat from a giant rat.

When confronted, he removes his covering and exposes himself as Sluggox, the sole survivor of an Ilithid colony that used to exist below Blue Harbor. The Sovereign and its thralls laid seige to the colony and then wiped it out once the mind flayers were sufficiently weakened, killing the Elder Brain.

Now free from the influence of the Elder Brain, Sluggox is starting to experience human emotion for the first time. He is starting to experience memories from his prior life as human named Silas. He regrets his actions and his need to feed on brains. He feels he has been reborn, and wants to experience humanity and emotions and friendship. He is fiercely devoted to Marne, the only friend he has ever known. Sluggox feels lonely cut off from the hive mind.

He has started capturing rats and subjecting them to psyionic blasts to create the cranium rats. His initial plan was to feed on them, but they were of too little intelligence. The sustenence he got from them did not surpace the energy required to create them. His next plan, reluctantly, is to use them to lure prey to him to feed on. He doesn't want to kill, but he doesn't want to give up on his fledgling humanity.

Possible outcomes:

1. Combat: the players can fight Silas and kill him or force him to flee

2. Alliance: the players can decide to find some way to bring Silas a fresh brain from an intelligent creature. Silas is open to partnership, and will offer his skills and scientific expertise as resources to the party in exchange.