The following ruleset was taken from the How To Host A Dungeon Wiki and was originally created by Dyson Logos over at Dyson's Dodecahedron. I heartily recommend  giving his blog a read as it is rather impressive and he draws some wonderful maps.

How To Host a Dungeon 

Muck Dwellers are one of many underground humanoid civilizations that are based around underground waterways, lakes and oceans such as the chaotic Kuo-Toa. They possess cruel and alien culture and art, with little science and practically no interest in hedonistic pursuits.

Setup

Draw constructions of the Muck Dwellers in green. Muck Dwellers prefer round and ovoid shaped rooms and curving passages near underground water.
If you don’t have an underground river or aquifer, roll on the following table for the appropriate water source to be added:

Roll	Water Source
1-3	Open Aquifer: Draw a cave 2 beads vertically in size, half this is taken up by water.
4-5	Closed Aquifer: Draw a cave 2 beads in size, filled with water.
6-7	Underground River: Create an underground river as per the Primordial Events table.
8	Underground Lake / Ocean: One one side of the map draw a cavern two thumbs in size that extends off the map filled with water and then continue as per the underground river from the Primordial Events table.

Muck Dweller represent soldiers, nobles and priests, while represent food, minor treasures (such as beads and pearls), tadpole muck dwellers and artworks. Muck Dwellers require to support their civilization.
Pick a spot adjacent to a water source for the Muck Dweller city. Draw two living quarters for and one tadpole pond for – as many as possible should be in direct contact with the water source. Place 2 and 1 in these chambers. This is the city of the Muck Dwellers – give it an interesting and hard to pronounce name like “Bloop-dil’ubloong” and write it on the map in pencil. It is now year zero of the Muck Dweller civilization.

The Muck Dweller Year

Any time you move beads from the city to a pool, you must leave at least 1 and 1 behind. If you can’t do this, don’t move the beads. If the Muck Dweller city is depopulated without a civilization-ending calamity, the largest pool attached to a water source claims one treasure from the ruins (or 1 if the city had no treasures) and becomes the new city.

Spring

In the spring, Muck Dwellers reproduce and care for their tadpoles. If there are at least two more than in the city or pool, remove 1 and add 1 .

Summer

In the summer, Muck Dwellers harvest the bounty of the water and raise their tadpoles. Add 1 to the city or pool if it is attached to a source of water. Muck Dweller cities or pools within one thumb of a source of ore or gems will dig out a mine the size of one thumb to collect 1 of ore, or collect 1 of gems.

Fall

In the fall, Muck Dwellers travel to spread the word of the fish gods. If the city has 2 and 2 or more, Muck Dwellers work to expand the empire.
If the city is on a river, they travel until they are at least one finger length down the river from the nearest city or pool and create a new pool there using 1 and 1 from the city (they may travel further if this gets them to a source of ore, as no two cities or pools can start within a finger length of each other).
If the city is not on a river, but there are other sources of water on the map (such as a river, lake or aquifer), then the Muck Dwellers dig a tunnel up to one half-finger length towards the water source.
If the city has the population to expand but is no more room for expansion on the water and no mining to be found within 1 thumb of any existing city or pool, the city becomes stagnant and loses 1.
In either case, if an expansion group of Muck Dwellers passes within a thumb of gold, mithral or gems, they will stop and immediately create a dry pool at that location to mine the ore or gems.

Winter

In the winter Muck Dwellers retreat into their cities and build. Tally the population of the entire Muck Dweller civilization and draw in any Muck Dweller Constructions marked at that population level that have not already been built – they can be built at either the city or the pools, at your discretion.
If there are more than in a city or pool during the winter, remove one .

End of the Muck Dweller Age

If the Muck Dwellers dig a tunnel to the surface, they panic at the sight of the sun and retreat under the leadership of their priests, abandoning their ‘contaminated’ cities and pools.
If the Muck Dwellers dig a tunnel beyond the bottom of the map, they discover things that were never meant to be uncovered by such lowly slime-covered creatures and are destroyed.
When the Muck Dweller Age ends, remove all Muck Dwellers and minor treasures and tadpoles from the map. In each city and pool, place 1 (Tadpoles that grew to maturity without the guidance of their culture and religion).

Muck Dweller Construction

Pop	Muck Dweller Construction
3	Breeding Pond. Draw an ornate room slightly larger than one bead that connects to both a room containing a and a (either directly or through tunnels).
4	Tomb with 1 Treasure (1 that does not count in population checks)
5	Chamber of Secrets. Create a two or three bead room one thumb away from the nearest Muck Dweller construction with no linking tunnel (it’s a secret). It contains essential religious secrets that are valueless to other civilizations.
6	Exploratory tunnel one half-finger long, roll d12 for clockface direction of exploration.
7	Temple of Mother Hydra. Create an ornate temple containing 1 (tadpole) and 1 that is a religious artefact and doesn’t count in population checks.
8	Dig an exploratory tunnel upwards one finger from the highest point of Muck Dweller constructions.
9	Temple of Father Dagon. Create an ornate temple containing 1 (priest) and 1 that is a religious artefact and doesn’t count in population checks.
10	Dig a deep shaft well down to the bottom of the page.

Revised River table (using d12 instead of D10)

Roll – Underground River
 1-3 : The river moves 1 finger horizontally across the page.
4-5 : The river changes angle sloping upwards while continuing across the page.
6-7 : The river changes angle sloping downwards while continuing across the page.
8-10 : The river moves 1 finger horizontally across the page, with a natural cave 1 bead in diameter in the middle of it. If the river is within 1 thumb of the surface, draw a sinkhole connecting this cave to the surface.
11 : Waterfall, dropping 1 half finger straight down.
12 : Underground lake d6+1 beads wide, river continues from other side.
This doesn’t drastically change the table or it’s results, but adds the underground lake.
More of a change comes in the Revised Natural Cavern table using a d8 instead of a d6:

Roll – Natural Cavern
1 : Draw a tunnel 1 finger long that passes through the cave
2 : Plague: Roll a d4. Write “plague” and the number rolled in the cave. Any group that encounters this cave will lose this number of (B)
3 : Gemstones: Draw a cave 1 bead in size. Roll a d4 and place that many (W) in the cave
4 : This is an empty cave
5 : Primordial Beasts: Place 1(B) in the cave. This represents primordial beasts that fight just like a wandering monster.
6 : Open Aquifer: Draw a cave 2 beads vertically in size, half this is taken up by water.
7 : Closed Aquifer: Draw a cave 2 beads in size, filled with water.
8 : Fate: Write “fate” in the cave. If any civilization tunnels into this cavern, the civilization will come to a natural end. If this occurs, the Fate cavern becomes a normal empty cavern.

For further variants on this and the existing civilisations please go check out  Dyson Logos post here or the How To Host A Dungeon Wiki.

Curious about How To Host A Dungeon? here is a link to last month's post.
Need more Civilisations? Here is my very own Kobold Rules.
The Great Spider Civilisation