The Siren of the Sewers.

The Lamplight Grotto

• If pressed they will begrudgingly mention a fellow siren named Flow with disdain who lives in the sewers now and may be in danger. Flow was once a beautiful mermaid who lived with Una and Waverly, but has since disappeared and isolated herself. Turns out they all had an argument as she is in love with a lowly fishmonger from the floating Market and was given a Stone by a witch that made her sing even more beautifully than the other Sirens so she could win his affection.

• The Body was local fisherman Goonter Hooklip, the mermaids know him to usually mind his own business by the river, but he would sometimes trade them interesting things he found for fish when he had a bad day. "He was all chewed up like Una's fish Bones" - Waverly.
• A small amount of pirate gold can be found in the shallows with a successful perception check.
• They will offer a small barnicle-covered chest of Pirate gold as a reward for helping them that was hidden in the grotto long ago. It contains A Chest of holding, about 16 liters of water, about 40 useable Imperial Era Coins, a vintage bottle of Cinntanian Reserve Rum dated 950, 2 gems, a small slooth suspended in a glass bottle, fighting an animated storm, and a pristine sextant and compass.

The Sewer Entrance Riddle.

• A clean marble statue of Doethine Goddess of Wisdom sits recessed within the rocky wall of the cave. She appears to be carved as if she is sitting on the ground with a pot held in her right arm all the while holding a weeping expression on her face. clear beads of water occasionally drip from the eye holes and flow down into a stream originating from a metal grate near the floor. An inscription is carved onto the pot, it reads: "My body is vast and I travel far. My life ends when my sorrows turn to tears.” The Answer is: CLOUDS.

As you say the word "cloud" the marble begins to animate itself before your eyes as the statue's weeping grimace turns to a subtle smile, the tears stop and for a moment you think she might have even winked at you. But the whole rock face begins to move and slide itself sideways revealing a stone passage beyond. Following it for a hundred feet or so, the stone surrounding you gives way to neatly laid bricks, and the shallow slapping of your feet in the babbling water becomes labored sloshing as the floor becomes a sunken gutter, the air slowly becoming thick with a miasma of ammonia and feces. You are now definitely entering the sewer system.

 

Deeper into the Sewers.

• There is a possible hidden door leading into the creature's lair. DC 15 perception, DC 15 slight of hand, or DC 17 strength.
• Random "flushes" that knock you into sewage as you cross in front of smaller pipes, or a special "pet" that gets flushed (think alligator, or something magical) DC10 DEX saves on slippery walkways to avoid falling into the slurry.

• 1-2 d6 Giant Rats.
• 3-4 Garum The Rat Catcher: This peculiar character hails to you in a muffled voice, their face concealed by an ugly leather mask. He pulls the mask up off his face revealing stark blue eyes and young boyish features. "I am Garum the Rat Catcher, what in Goddes's name brings you folks down here? You picked a terrible time to visit if there is a good time to visit the sewers. May I ask why you're here? I hunt for profit and fun! Lord Elmer gives me 2 coppers a tail, and a silver piece for the large ones! But I'm going to see if I can negotiate it to two silvers because the Rats are getting bigger and meaner lately! And clearly its more dangerous down here now since I found that body!"
• 5-6 Infected Goblin fighting the Ratcatcher. "The party sees a man embroiled in a fight with some unspeakably filthy and smelly goblins! One particular goblin has black thick blood that burns when touched, a failed constitution throw will result in poison, while another failure will result in Sewer Plague popping up in d4 days which causes exhaustion.

• An engagement ring that was lost down a drain worth 5gp.
• A "Poop knife", it's a dagger that deals 1d4 bonus poison damage. . . doesn't matter, it's a poop knife, and nobody wants to get stabbed by that!
• Pinecone of freshness: Enchanted by druids and sold to nobles, this pinecone on a string loop is hung in the privy, or just inside a sewer entrance, to prevent foul smells from offending the weak nobles' noses. The strings break easily though so there are a few lost into the sewers, some of which whose magic is not yet overwhelmed by the stench. These pinecones cleanse the air in a 15’ cube centered around them, giving all within advantage on saves vs airborne toxins and stench effects.
• Toadstools of noxious will: These disgusting yellow gooey fungi can be found in the sewers by the very lucky. They do not look edible and smell worse than the sewer they are found in. A survival check ( DC 16 ) identifies them for what they are. However, if swallowed the eater spends their next round vomiting the contents of their stomach and falling prone, but as they recover at the start of the next round they will realize the pool of vomit is quivering and moving, and rising up into a vaguely humanoid shape. The toadstool vomit uses the stats of a oblex spawn and is completely loyal to its creator, and can be commanded with a bonus action. The spawn loses coherence and becomes a nonmagical vomit pool after 2 hours or if it runs out of HP.

The U-Bend.

• DC 15 Con save if they swim through it, for sewer plague.

The Lair.

• DC 5 perception check
  Suddenly another sound catches your attention and overpowers the singing, the beautiful voice replaced by a sickly hissing and rhythmic heavy wet slapping sounds coming from the closest room

• Sahuagin pg. 343 PHB

• Lair Action: The beast will tear at pipes around it in a fit of rage, releasing fountains of poisonous liquid and gas. It may also dive into the sewage and reappear elsewhere when its health is low.
• Loot: A soiled necklace made of braided colored string and old Imperial coins, and a single ordinary-looking stone in the center bearing a rune (protection). DC10 insight/history check to know that it is a common custom amongst the river folk to carry stones from the old island of Cinn. DC 15 perception And engagement band has grown into the webbing of its hand worth 3gp.
• Next to a strange mass of what almost looks like tissue or paper you find a round half-empty bottle of Scute's Snake oil "melts your skin right off!" can be used as an acid vial and does 2d6 acid damage.

The Sirens Cistern.

• DC 16 Intelligence (Investigation) checks to discern that the Siren is disguised.

• She will use Charm by singing DC 10 wiz save to resist Charm.
• Lair action: She will turn a wheel to begin flooding the cistern with water that will fill the area in 2+d4 turns all ground becomes difficult terrain within 1 turn.
• Loot: A stone and knuckle bone necklace, that glows with strange purplish ripples of energy within. Holding it allows the wearer to sing beautifully but they will notice it greatly strains their normal speaking voice. If worn for an extended period of time it will need CON saves and begins to corrupt the wearer much like the mermaid. An engagement silver wristband decorated with images of sailing ships worth 30sp. In the rubble around there is a half a jar of Ram's Restorative Ointment; it has 1+ 1d4 uses and as an action, one dose of the ointment can be swallowed or applied to the skin. The creature that receives it regains 2d8 + 2 hit points, ceases to be poisoned, and is cured of any disease.
• DC 17 check for a hidden passage to the cistern connected to Archies Aleworks.

• Lowden The Rat Catcher periodically patrols the sewers under the streets of Westlight, he has long been the local rat catcher, but lately he has spent more and more time underground sighting an increase in the Rat population "Seems no matter how many I bag, their numbers recover soon returning with bigger and angrier vermin!" Lowden saw Flow exit the sewers and post the notice, he was actually a bit offended she didn't come to him, but he understands the situation is getting dire. He was actually about to seek out help himself!

• Una, and Waverly are two Sirens that hang out at the entrance to the Grotto. They know about the rats but reveal they are only a problem in the actual sewers, which they avoid when they can, but they didn't know about the notice. If pressed they will begrudgingly mention a fellow siren named Flow with disdain who lives in the sewers now. Flow was once a beautiful mermaid who lived with Una and Waverly, but has since disappeared and isolated herself. Turns out they all had an argument after she was given a Stone by a witch that made her sing even more beautifully than the other Sirens.