Originally published in Worldbuilding Magazine (Volume 2, Issue 4) “Creatures”, p 18.

If you have ever wanted to see inside the mind of a game master (GM) while she creates a fantasy ecosystem for her players, this is your chance. Be warned that my mind is not a formal place. There will be chattiness, false starts, random allusions, and mildly crude humor. Skip to the end for a concise list of techniques and creatures covered. Otherwise, continue on.

I don’t remember when I first got the idea to put the Great Fens in my world, but I do know why I did it. It was the name. Some names are just ripe with ambience. Patagonia. Kathmandu. The Great Fens.

I had a vague feeling that fens were some sort of temperate inland wetlands, but I didn’t have any details to go with that impression. Nonetheless, they loomed large in my imagination as an impossibly gothic landscape. Like the isolated moors of a Bronte novel. But soggy. And without the excruciatingly proper Victorian manners.

So when I created my campaign world, I put my own Great Fens in the far southern hemisphere. I eagerly anticipated the journey my players would experience crossing this moody region. When it came time to actually prepare for their adventures in the Great Fens, I confidently began looking in the usual places – monster manuals and bestiaries and subreddits.

They failed me.

Although “The Great Fens” sounds evocative to my ear, fens aren’t commonly used in monster manuals and bestiaries. The RPG content creators like to focus on forests, swamps, cities, caverns, deserts, mountains and oceans. Those are the popular biomes, with a bounty of specially designed monsters and descriptive articles. Stories and modules set in those beloved ecosystems are lushly described with inspiring detail. But fens? Not so much.

I thought perhaps I could just use swamp creatures, but I thought I should check to be sure.

The Research

The happy thing about living in the future (unevenly distributed) is that curiosity and ignorance are so very easily satisfied. So my next step was clear. To the internet!

According to the US Forest service website, “there are several types of wetlands: swamps, marshes, bogs, and fens.” Here’s how to tell the difference:Swamps are dominated by woody plants. A canopy makes them distinct in feel and population from the other wetland types. Also, they’re mostly nearer the tropics and warmer temperate regions.

Marshes may be tidal salt or freshwater marshes which are inundated a couple times a day by the tides, or they may be inland freshwater marshes. In either case, they form on the boundaries of true water ecosystems – rivers, lakes, and seas. Some marshes are seasonal, drying out on occasion. All are dominated by herbaceous (non-woody) plants.

Bogs are a type of peatland where the plants die faster than they can decompose and slowly accrete into peat. The soil is starved of oxygen, so you end up having different plants and animals with marshes. Also, the word “bog” sounds more miserable than marsh, like bogs just have to have a greater mosquito/fly density based purely on consonant sounds.

Fens are peatlands like bogs, except the water comes up through the ground rather than through inundation or precipitation. They’re less acidic and have more nutrients than bogs. They’re mostly found in the far north.

So, I had my first answer. No, fens are not like swamps. In particular, I definitely couldn’t use most of the RPG swamp monsters I had unthinkingly assumed would work. No reptiles or amphibians, because fens are too cold for them, especially in late fall when I wanted the adventure to take place. Maybe a cold dragon would have worked, but not for my first-level party. It’s not really the right place for vine monsters or living trees either, because the landscape of a fen is tree-deprived and the vines need trees to climb on.

So, that’s what I couldn’t use, but how to find what I could? To the internet, again!

Except this time I looked more at travel/visitor web sites. I found an inspiring one at the Great Fen Project. It has evocative descriptions of what a person would experience if they visited the fen at a different time of year, including objective information on local animals and weather, as well as subjective sensory impressions. For example:

“Winter can be a very good time to see wildlife on the Great Fen. Winds can be harsh as they sweep unhindered across a vast bare landscape, but on clear days, with the sun low in the sky, colours seem brighter and clearer. The flocks of wintering wildfowl on the meres in Holme and Woodwalton Fens become larger and more vocal: listen for the whistle of Wigeon, sometimes drowned out by the yelping of Greylag Geese.”

Now that’s ambience.

For my campaign, I took these beautiful descriptions and edited and repurposed them so that each adventuring day, I read the players a quick sentence or so to give them an idea of what they were experiencing. For example:

“Day 1: Although you’re not too far from Greentongue keep, the winds are harsh as they sweep unhindered across the vast bare landscape. In the distance, you hear the yelping flocks of geese wintering on the meres.”


“Day 2: Dawn rises late, but the colours in the sky seem brighter and clearer in the bitter cold air. Rabbits scurry through the straw-colored grass.”

The Real Ecosystem

I now had a general feel for the fens. By looking at a number of sites, I put together a picture of the real life critters and plant life that inhabit the fens, as well as details of climate and landforms.

There would be both small mammals – mice, voles, and rabbits – and larger mammals such as badgers, water deer, foxes, and otters. The skies would be filled with birds: everything from the seasonal visits of the migrating swans, ducks, and geese to the tiny wrens, treecreepers, and goldcrests singing in the trees and bushes that occasionally dot the landscape. Hunting from above would be the hen harriers and barn owls.

But the land is defined by water, so when I planned out the animals, I had to think of the eels. Apparently they were a cornerstone of the dark age economy of the British Fens, being “used as currency and for payment of rent.” Eels made me think of lampreys which are the most terrifying of all the wiggly organisms (in my mind that’s a clear and useful category that includes eels, snakes, caecilians, and worms). Lampreys are creepy enough that I clearly needed to include them in my census to have a properly monstrous Great Fens.

Not all the species in the water would be horrible, though. The fens would be home to fish such as bass and perch and pike and bluegill. There would be snakes and frogs and mosquitoes, but not for my players to see; the real amphibians and reptiles would be hibernating during the late autumn session.

In terms of plants, fens are dominated by water-loving grasses, reeds, and rushes. There would be wildflowers in spring perhaps. A few berry-bearing bushes such as hawthorn and blackthorn would be mostly picked over. Flowering ash could perhaps dot the sparse patches where the ground stayed firm. Heath and bog myrtle and sphagnum mosses could live in the firmer ground. Fungi of every kind would fill the landscape: familiar mushrooms, spreading mold, coral-like structures, and shapes too alien to be easily named. And below everything would be peat.

But I was writing an RPG campaign, not creating a scientific study, so I could also put in creatures that don’t exist in the real world. The trick to making the ecosystem feel cohesive is in using real-world details for inspiration.

Bringing in Extinct Species

Since I was creating a fantasy ecosystem, I didn’t have to limit myself to the current-day species that inhabit the fens. In fact, I didn’t want to. So I broadened my net to include species which could have lived there in some alternate dimension where extinct species coexist with modern day ones. And my first sources of inspiration were the fungi.

Since the descriptions of fens made them out to be fungi heavens, I thought of prototaxites from the Silurian and Devonian periods. I had run across a picture of them in my twitter feed the week before, dotting the landscape like living monoliths. The idea of 10-foot tall fungi impressed me greatly, so they had stuck in my mind. I decided they were definitely in my fens. Giant phallic fungi for the win.

And that made me wonder what other critters existed alongside the prototaxites.

A quick Google search led me to the formidable sea scorpion. It wouldn’t really have survived in my chilly Great Fens because its normal habitat was a warm, salt water environment, but I loved the look of it. So I waved my World Builder’s Wand™ and created the ever-so-slightly smaller and more cold-hardy fen scorpion. I reasoned that lobsters do perfectly well in cold waters, and sea scorpions were related to both lobsters and real scorpions, so I could see a fen version having slightly more lobster-ish front claws and doing well in the colder fens. Real sea scorpions grew to 8 feet long, but I’d be reasonable and make the fen version a piddly four-to-six feet long.

And there were also lots of insects – butterflies, dragonflies, mosquitoes, millipedes, and centipedes, but I couldn’t see them being part of the winter landscape. I made a note to include giant, creepy fen-i-pedes breaking out of the mud after spring thaw if the party ever came back in warmer weather.

Since we already have eels, I just had to look to see what horrible, ancient, eel-like creature was even more terrifying than the lamprey but is now extinct. Unfortunately the search didn’t turn up anything more impressive. Lampreys are just hard to top. I imagine even the sandworms from Dune are all, “Yeah, they’re small, but did you see the teeth on those things?”

The creature that really caught my eye was the Tiktaalik Rosae. It’s sort of an evolutionary link between the lobed fishes and the first of the tetrapods, popularly called a fishapod. Its amphibian-like ribcage allowed it to venture onto land. But what caught my eye was that it had the scales, gills, and fins of a fish but the head and neck reminiscent of a crocodile. Can you imagine a shark-o-gator that blends into the muddy fen banks but can follow you onto land when you try to get away?

Adding Magical Creatures

As I developed an understanding of fens as a natural ecosystem, I also started to think about the magical creatures. An easy place to start was the elementals, which are easy to find among RPG monster resources. I just chose a couple that looked likely – small enough to live in the shallow waters, ornery when attacked, and at home in still or slow-running fresh water. Mud and fungi creatures are a bit less common in published RPG resources but still plentiful. Moss monsters are even more rare, but it’s easy enough to repurpose generic plant or ooze creatures to have a sphagnum or brown moss flavor. They could have a spore attack, probably one that messed with the mind or made breathing difficult.

I was setting my adventure in the subarctic with winter drawing close, so I imagined magical creatures of ice and cold migrating just behind the geese and swans. As the weather cooled enough to support them, these elementals could spread out to breed or eat. Cold elementals are common in published resources, so populating the land was just a matter of deciding on a couple favorites.

Although some days are clear, I imagine sometimes there is fog and mist rising from all that water, especially when the weather turns a bit warmer. So, there could be creatures swirling in those mists, fooling the players’ senses and feeding off the energy of the living. Fog is inherently creepy. The fens are inherently damp. Therefore, through logic, a creepy, damp place like the Great Fens would just have to have fog.

When I populated my mythical landscape with people, I used the ancient Welsh as a source of inspiration. So I read up a bit on Welsh folklore to see what they had that could work in the fens.

A few legends especially caught my eye. The first was the Afanc, a lake monster variously resembling a crocodile, a beaver, or a dwarf (depending on the source), but which is generally considered to be a type of demon. I already liked the idea of the prehistoric shark-o-gator, the Tiktaalik Rosae, but Afanc is a much cooler term, so I combined the two ideas. My Afanc would look like the prehistoric version, and would live in the shallow mud and act as an ambush predator. And like a beaver, it would be a builder – making traps to help catch its prey. I wanted it to still be a natural creature, but the demon aspect was interesting. So I decided that the species is particularly susceptible to getting possessed by demons. The possessed version casts illusions to make its traps harder to spot, and killing it only makes it pause before the demon fully takes over the corpse. Thus my demon-susceptible, trap-making shark-o-gator Afanc was born.

The Cŵn Annwn also seemed perfect – they’re spectral hounds with red ears, associated with undeath, the wild hunt, and migrating geese. I reflavored ghost wolves, a monster from the D&D 4th edition compendium, to fit the description. My new version looked somewhat like a wolf or hound but was undead and insubstantial. They were especially aggressive in the fall, following the migrating geese. They had a necrotic bite, and they blended into the mists to help hide them in their hunt.

The Cyhyraeth or Hag of the Mist, was another mythological creature that I liked. In folklore, she’s a harpy-like wraith, mostly invisible but appearing out of the mists at crossroads or in streams to foretell your death. I combined that idea with one of the higher level undead mist elementals to have a Mist Hag that appears in foggy conditions. She can change her shape to mimic the appearance of anyone whose death she saw, using trickery and guile to lure travelers close before feasting on their fear and life force.

Like many cultures of the British Isles, the Welsh also have a variant of the phooka. Shapeshifters associated with water, they can be helpful or harmful or tricksy, depending on the story told. They are always intelligent, though. I felt like they belonged in the fens, but I didn’t have a good encounter for them yet, so I reluctantly left them out of my planning.

Another one I didn’t have a good place for was the Adar Llwch Gwin. They are giant birds that understand human languages. I kept them in mind in case we ever came back to the Great Fens, but decided to treat them as exceptionally rare.  My players didn’t see any in this adventure.

The Takeaway

Now, my guess is that you won’t have an immediate need to create an adventure in the fens. Few people do. But fens are hardly the only ecosystem so underserved. You might want to be inspired by the deserts of the American southwest or the Australian outback rather than the Sahara. Perhaps the spiny forests of Madagascar or the bamboo forests of Japan whet your interests rather than the more commonly used European forests.

Regardless of the details, if you want to build a fantasy ecosystem that is inspired by a real one, here is the quick take on my technique:

  1. Pick a specific place and use search engines, studies, or national park sites to familiarize yourself with it. Go to your library and see what resources are available. Take notes.
  2. Especially note the species, climate, and landforms that characterize it.
  3. Try travel sites for evocative descriptions. Try the weather channel or a similar site to get a feel for typical weather at a particular time of year.
  4. Research extinct species in similar climates to inspire creation of new creatures.
  5. Look up magical species in RPG bestiaries and monster manuals, focusing on ones that have elements you recognize from your research.
  6. Choose a culture, preferably one that inhabited the area you chose, and research its folklore.

And if you do want to set your own adventure in the fens, here are some critters with which to populate it.

TypePlants/Fungi (W)WaterLandAir
Flowering Ash
Sphagnum Moss
Brown Moss

Millipedes (S)
Centipedes (S)
Frogs (S)
Snakes (S)
Water deer

Butterflies (S)
Dragonflies (S)
Mosquitoes (S)
Hen harriers
Barn Owls
Moss elementals (S)
Fen scorpions
Water Elementals (S)
Ice Elementals (W)
Mud elementals (S)
Adar Llwch Gwin
Cŵn Annwn
Mist Hag