Sewer

The vaulted drains under a city, where its waste and rainwater run to the river. It is dark, wet and stinking, with narrow ledges beside deep channels and grates that open onto the streets above. Thieves, smugglers and the city's outcasts use it as a road, and things that like filth grow large down here.

Light

Dark

Going

Hard

Hear it

• Sewers: Underground Sewers (In the album RPG Ambiences Vol. 1)
• By boat: Rowboat in Sewers (On Patreon, Raven tier)
• Thieves: Sewer Hideaway of Thieves' Guild (In the album RPG Ambiences Vol. 1)
• Cultists: Sewers with Monsters and Cultists (In the album RPG Ambiences Vol. 6)
• Zombies: Zombies Infested Sewers (In the album Modern Ambiences Vol. 2)
• Mushrooms: Underground Mushroom Forest in the Sewers (On Patreon, Guardian tier)
• Deep inside: Sewer Leading to Caverns (On Patreon, Guardian tier)

Hazards

• Darkness. Only the grates let in a little grey light. Between them, nothing but the sound of water. D&D 5e: The tunnels are Darkness, Heavily Obscured: a creature trying to see into them has the Blinded condition. Under a grate in daytime there is Dim Light, where Wisdom (Perception) checks that rely on sight have Disadvantage. Darkvision treats Darkness as Dim Light within its range.
• Foul air. Pockets of gas collect where the channel bends. They make the head spin, and a naked flame can set them off. D&D 5e (House rule): Our rule: a creature that enters a gas pocket makes a DC 12 Constitution save or has the Poisoned condition for 10 minutes. If an open flame enters it, the gas flares: each creature within 3 m (10 ft) makes a DC 13 Dexterity save, taking 2d6 Fire damage on a failure or half on a success. A closed lantern is safe.
• Slick ledges. The walkways are narrow and coated in slime. One slip and you are in the channel. D&D 5e (House rule): Our rule, after the SRD's slippery ice: the ledges are Difficult Terrain. A creature that dashes or is shoved there makes a DC 10 Dexterity save or falls Prone into the channel.
• Flood surge. When it rains in the city above, the channels rise in minutes and the current sweeps everything to the outfall. D&D 5e (House rule): Our rule: a creature caught in the surge makes a DC 13 Strength (Athletics) check each round or is carried 6 m (20 ft) downstream and takes 1d6 Bludgeoning damage. Holding a grate or a rope gives Advantage. Its breath runs out as the SRD's Suffocation says if the tunnel fills.
• Filth fever. Every cut gets dirty down here, and a dirty cut turns hot and red by the next morning. D&D 5e (House rule): Our rule: a creature that took Piercing or Slashing damage in the sewer, or swallowed its water, makes a DC 12 Constitution save after its next long rest. On a failure it has the Poisoned condition and regains only half the Hit Points from rests until cured by a DC 13 Medicine check, a Lesser Restoration or a day in bed.

Adventure hooks

1. A mudlark sifting the drains for lost coins found a ring bearing a councillor’s seal. The councillor wants it back quietly, and the mudlark has already sold it to someone who does not want it back quietly.
2. Smugglers use the old drains to bring goods in under the walls. A storm surge is coming down from the hills tonight, and their cargo this time is a family who could not pay the gate.
3. A rat catcher has trained the sewer rats to follow his whistle and wants to sell the trick to the guild. The rats have started following other whistles too, including the watch’s.
4. Something under the tannery has grown large on its waste. The tanner wants it killed before the city finds out what else has gone down his drain.

What may happen

1. Through a grate above, two people are planning a robbery for tonight.
2. A flat boat loaded with casks comes down the channel, poled by two silent men.
3. The squeaking grows to a roar: a river of rats pours along the ledge toward the party.
4. Something glints in the silt of a sump. Reaching for it means putting an arm in to the shoulder.
5. A child with a sack and a stick searches the silt for scraps. She knows every tunnel and charges a copper a turn.
6. The brick gives way to older stone, cut long before the city was built.
7. Thieves' marks in chalk at a junction: one means safe, one means a toll, one means run.
8. A body floats past face down, still wearing good boots.
9. An old man lives in a dry side chamber with a dozen cats and knows what lives in the deep cisterns.
10. Bubbles rise in the still water of a cistern, in a slow line, as if something were breathing below.
11. A doorway bricked up in a hurry, with a cellar on the other side.
12. The temple bell rings above. Down here it shakes the water and drowns every other sound.

Creatures

• Cultist (a threat to the unwary, Challenge 1/8-2)
• Giant Rat (a threat to the unwary, Challenge 1/8)
• Giant Centipede (a threat to the unwary, Challenge 1/4)
• Swarm of Rats (a threat to the unwary, Challenge 1/4)
• Gray Ooze (a threat to the unwary, Challenge 1/2)
• Ghoul (a fight to fear, Challenge 1)
• Giant Toad (a fight to fear, Challenge 1)
• Gelatinous Cube (a fight to fear, Challenge 2)
• Ochre Jelly (a fight to fear, Challenge 2)
• Wererat (a fight to fear, Challenge 2)
• Black Pudding (a fight to fear, Challenge 4)
• Otyugh (deadly, Challenge 5)

To bring back

Rat Heart, Lycanthrope Fur

Other sites

Dungeon, Crypt, Graveyard, Ruin, Fortress, Mine Works, Temple, Laboratory, City

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

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