Flowstone – an underground city for the Black Hack, or any OSR game

Flowstone is a city for my own Undergloom setting (previous post here), though it could most likely be plugged in to any Underdark-style world. I use it for the occasional Black Hack game, but it should work with any OSR-style system.

Flowstone lies in the center of a vast cavern that serves as a junction of five heavily traveled Undergloom trade routes. Think of it as an underground equivalent of a Silk Road trade city.

The city has roughly circular shape, surrounded by a 20′ high stone wall. Five gates allow access, one for each tunnel. In the center of the city is a large steomite (a rocky column where a stalagmite meets a stalactite), that reaches to the ceiling of the cavern. It’s commonly referred to as the Tower. Numerous dwellings have been carved in the sides of the Tower. A road has been carved into it, spiraling up the tower to the cavern ceiling. Generally, the more prominent a city resident is in Flowstone, the higher up on the Tower is their residence.

Flowstone is ruled by a mysterious being called the Lightless Monarch. The Monarch has never been seen in public, but it’s widely assumed she is a Chiropteran (a bat-like humanoid). She dwells in the Royal Roost at the top of the Tower, protected by the Noctillionine Guard. 

The Noctillionine Guard is Flowstone’s army and police force. It’s made up entirely of Chiropterans – flying bat-like humanoids. They also man the walls around the city.

Flowstone’s economy depends on the many trade caravans that pass through the city. It’s a safe place where all the different races and civilizations below the earth can meet and trade.

Entering the City

Anyone is allowed to enter Flowstone, but they’re required to pay two small fees first:

Illumination Fee: For the benefit of the Flowstone’s less nocturnal inhabitants, there are multiple tethered floating balloons above the city that provide a dull white illumination. They contain weak, warded fire spirits. For this service, anyone entering the city is required to pay 2 coins, whether they need the light or not.

Arterial Tax: The Lightless Monarch has decreed that any being physically capable is required to make a small blood donation upon entering the city. No one’s quite sure what the blood is for, but it is all collected and brought immediately to the Tower. Anyone making a donation will lose 1 HP for a day.

Places of Note in Flowstone

The Royal Roost: The palace of the Lightless Monarch at the very top of the Tower. The only access is by air. Heavily guarded by the Noctillionine Guard, who are often seen flying to and from the tower. Only a few of the most powerful figures in Flowstone have ever been allowed to enter the Roost.

The Crevasse: A jagged chasm in the floor of the cavern. The Crevasse, and the shanty neighborhood around it, have a horrible stench. The people of Flowstone use the Crevasse for waste disposal. Garbage, waste, and the occasional inconvenient person, all end up being dumped here. No one’s entirely sure what’s in the Crevasse, but scavengers occasionally descend into its depths to recover lost items.

The Marketplace: The economic heart of Flowstone. Caravans from across the Undergloom meet here, trading goods of every description. Deep Gnomes, Gloom Elves, Troglodytes, and other strange denizens of the lightless world all trade here peaceably, watched over by the ever-vigilant Noctillionine Guard, who patrol the sky above the crowded market stalls. Local merchants provide all of the gear, livestock, and supplies any traveler in the tunnels might need. The Lightless Monarch charges a small fee to anyone doing business in the Marketplace.

Reservoirs: Dripping water from the cavern ceiling is collected in this pond. Water is a valuable resource in Flowstone, so the reservoir is well guarded.

Speological Society: A meeting hall and private social club for adventurers and explorers of the Undergloom. Dues are 5 coins a month. Membership is by invitation only. Excellent place to make contacts, hear (and spread) rumors, and pick up all kinds of information. Their library is supposedly one of the most extensive collections of knowledge in the Undergloom.

Lithospheric League-house: The Lithospheric League, the society of merchants from the surface world, maintains this compound for its employees. It provides lodging, food, and serves and a mail depot. Messages can be left for other League members here, even ones on the surface. The messages won’t travel quickly, but they’ll eventually get there. League members can also apply to join the various trade and exploration missions the League is constantly mounting.

Webbed Cathedral: A temple to the spider goddess of the Gloom Elves, carved into the lowest level of the Tower. Its primary use is to serve the spiritual needs of the Gloom Elf caravans, but the priests of the cathedral are actively evangelical, trying to tempt others into worshiping their dark goddess. A bell in the cathedral strikes every hour, audible throughout the city.

City Walls: Since the walls are manned exclusively by flying bat-people, they have no stairs or ladders, and the walls and towers are only accessible by air.

Things to do in Flowstone

  • Scale the Tower to steal the treasures of the rich.
  • Haggle in the Marketplace
  • Spin tall tales with the adventurers of the Speological Society
  • Descend into the Crevasse, seeking the lost and the forgotten.
  • Look for caravan work with the Lithospheric League

Rumors in Flowstone

  • Deep within the Crevasse, a powerful ratkin dubbed the Duke of the Down Below has carved out a small empire for himself.
  • The Arterial Tax is used to feed the Lightless Monarch, who is actually an ancient vampiric, possibly demonic, beast.
  • The priests of the Webbed Cathedral are planning to summon something horrific – something that will either give them control of the city, or destroy it completely.
  • A terrible beast dwells within in the Reservoir, but the city government is too afraid to attack it, afraid they’ll poison the city’s vital water supply.
  • A bold thief (or band of thieves) has been staging wildly daring robberies of the richer merchants in Flowstone. No public announcement has been made, but supposedly the Noctillionine Guard has put a huge price on the perpetrator’s head (or heads).


One of the most common races of the city, the Chiropterans are humanoid bat like creatures. They make up the Noctillionine Guard that defend the city and uphold the laws, but also hold many of the city’s civil service positions. Many of the wealthier noble families on the tower are Chiropterans as well. Most of them are capable of flight, and all of them have sonar to see in the dark.

Noctillionine Guard – HD 2
Halberd – STR (1 Nearby) 4 dmg
Crossbow – DEX (1 Distant) 3 dmg
Flutter – Once per battle, when a Character hits the Chiropteran, they are forced to re-roll the Attack.
Nocturnal – Can see in the dark using sound waves. Silence spells will blind them.

Chiropteran Alchemist HD – 1
Dagger – STR (1 Nearby) 2 dmg
Acid Bomb – DEX (1d4 Nearby) 4 dmg
Nocturnal – Can see in the dark using sound waves. Silence spells will blind them.

Chiropteran Noble – HD 3
Blade – STR (1 Nearby) 3 dmg
Loyal Guards – Can summon 1d6 Noctillionine Guards
Nocturnal – Can see in the dark using sound waves. Silence spells will blind them.

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