Bringing Cosmic Horror to 5e: The Making of R'lyeh Rising

I created R’lyeh Rising to bring H.P. Lovecraft’s cosmic horror to the most widely played and understood TTRPG system: 5th Edition. My group predominantly plays 5e, but I wanted to explore investigative horror in a more contemporary 1920s setting without forcing everyone to learn an entirely new ruleset.

Other Mythos RPGs often felt either too mechanically complex, with rules that got in the way of play, or transplanted Cthulhu Mythos creatures into a traditional D&D fantasy world. Neither captured the slow-burning, creeping dread of Lovecraft’s stories.

R’lyeh Rising is designed to feel like genuine 1920s cosmic horror, not fantasy with tentacles and Great Old Ones, but investigative horror about mortal heroes, madness, the fear of the unknown and truths that should never be uncovered. The result is a complete Lovecraftian campaign setting built on rules familiar enough for anyone who knows Dungeons & Dragons to pick up and play.

What Makes R'lyeh Rising Different?

Creating a horror setting for 5e meant rethinking several core rules and assumptions. Here's how I adapted the rules to serve the genre:

Period-Accurate Equipment and Firearms

The 1920s was an era of dramatic technological change. Advances in firearms, telecommunications, automobiles and aviation transformed everyday life. To make the setting feel complete, I created detailed rules for everything from forensic investigation kits and Tommy guns to Model T Fords, aircraft and watercraft. Equipment is priced in 1920s dollars using historically accurate costs wherever possible.

Firearms received special attention. They required extensive playtesting to balance them against the 5e action economy while still making them feel appropriately dangerous. A shotgun blast shouldn’t be something a fighter can simply shrug off.

Armour presented a greater challenge. Fantasy settings offer centuries of established armour, but practical bullet-resistant protection was still limited during the 1920s. Plate armour was obsolete and Kevlar was decades away. Yet armor is a core part of 5e, both mechanically and as a way for players to improve their characters’ survivability.

To solve this, I drew on experimental armor developed during the First World War. Soldiers sometimes purchased their own protection and the major powers tested everything from medieval-looking German trench armor to America’s Brewster Body Shield, a massive steel suit with the appearance of a walking tank, and more than a passing resemblance to Marvel’s Juggernaut.

It seemed reasonable that some of this equipment might still be available through military surplus dealers during the 1920s, so it became the basis for the game’s heavier armor. Lighter options draw from the silk, cotton and metal-reinforced bullet-resistant vests that existed at the time. These could stop some lower-powered handgun rounds while remaining concealable beneath an ordinary suit or waistcoat.

Bullet-proof vest

Trench Armour

Vehicle Chase Mechanics

Car chases are iconic to the era.  Think bootleggers outrunning prohibition agents or investigators fleeing cultists down winding coastal roads. R’lyeh Rising provides simple chase mechanics that use contested Vehicle Handling checks, with faster vehicles gaining advantage. The rules are streamlined enough to run quickly but create genuine tension and a chance for either side to win regardless of the vehicles involved.

Ritual Spellcasting and the Madness System

Magic in R'lyeh Rising isn't common or safe. It's a violation of the natural order of the universe. The setting provides an optional ritual spellcasting system that allows casters to reach beyond their normal limits, but at terrible cost. When a ritual goes wrong, the Spell Backlash table delivers consequences ranging from temporary madness to summoning hostile entities.

The existing 5e madness system has been expanded significantly with new tables for short-term, long-term, and indefinite madness, and effects that create roleplay opportunities rather than just mechanical penalties. Madness also accumulates. Three short-term episodes become long-term; three long-term episodes become indefinite. This creates a resource players must manage carefully.

Cults, Creatures, and Cosmic Threats

R’lyeh Rising doesn't just provide monster stat blocks. It builds complete organizations that GMs can use as campaign-spanning antagonists. The Esoteric Order of Dagon, the Church of Starry Wisdom, the Cthulhu Cult—each has beliefs, practices, membership structures, and goals that make them the building blocks of entire campaigns.

Our creature designs emphasize horror over combat challenges. The Spawn of Cthulhu infiltrates human society using a reality distortion field that makes intelligent creatures perceive it as normal, only children and animals see the truth. The Spawn of Shub-Niggurath offers bargains that seem beneficial until the full curse manifests. These creatures create Storytelling opportunities.

Forbidden Texts as Magical Items

The Necronomicon, De Vermis Mysteriis, Unaussprechlichen Kulten. These infamous grimoires appear as detailed magical items with specific benefits and terrible costs. They require attunement and risk madness, but grant powers that may prove essential against cosmic threats. They're rewards, plot devices, and corrupting influences all at once.

Species and Classes: Authentic to Lovecraft

Fifth Edition has always embraced the idea that players should be able to choose from interesting species and classes. I didn't want R'lyeh Rising to be any different, but every option needed to feel authentic to Lovecraft's fiction and the 1920s setting.

That meant drawing directly from Lovecraft's stories for inspiration. Each new species ties back to something that appears in the original tales, adapted for player use while preserving the horror and strangeness that makes the Mythos compelling.

Playable Species

Deep One Hybrids are the cursed offspring of humans and the amphibious Deep Ones who dwell in sunken cities beneath the sea. They appear human at birth but gradually transform as they age, developing gills, webbed extremities and an irresistible call toward the ocean. Inspired by the doomed residents of Innsmouth in "The Shadow Over Innsmouth," playing a hybrid means confronting an inevitable transformation and choosing whether to embrace or resist it.

Mind Urns of Yuggoth are human brains preserved in alien cylinders by the fungoid Mi-Go, installed into mechanical bodies of extraterrestrial manufacture. Drawing from "The Whisperer in Darkness," these characters have sacrificed their humanity for a form of immortality, trapped between worlds and forced to conceal their true nature from a humanity utterly unprepared to learn they exist.

Serpent Folk are remnants of a prehuman reptilian civilization that once ruled the Earth. They survive in hidden enclaves or pass among humans using illusion and hypnosis. Lovecraft referenced serpent people in several stories, and they connect to the broader tradition of ancient, inhuman intelligences that predate humanity and view us as interlopers.

Umbrals carry fragmentary traces of outer god lineage diluted across countless generations. They don't descend directly from cosmic horrors but bear echoes of ancient unions between mortals and entities like Yog-Sothoth, Nyarlathotep or Shub-Niggurath. Their appearance varies wildly: strange skin tones, unusual eyes, vestigial growths or patterns of impossible geometry. Many find their way to carnivals and travelling shows where their differences become assets.

Classes from the Core Rules

Not every 5e class fits a 1920s Lovecraftian setting. Medieval knights, wilderness rangers and arcane wizards feel out of place when the goal is investigative horror in New England. I selected classes and subclasses that made sense for the era while providing enough variety for different play styles.

Warlocks are the most natural fit for Lovecraftian magic. In the stories, sorcery rarely comes from formal study or innate talent. It comes from reading the wrong book, making contact with entities beyond human comprehension or inheriting knowledge that should have stayed buried. Warlocks with the Great Old One or Fiend patron capture this perfectly: ordinary people who stumbled into power they don't fully understand and now carry the consequences.

Fighters are essential to any setting where violence remains an option. Veterans of the Great War, police officers, bootlegger muscle and private security all fit the archetype. The Champion subclass from the core rules works well, but I also created a new Gunner subclass specifically for firearms specialists. The Gunner handles the realities of 1920s combat: quick reloads, trick shots and the lethality of modern weapons in trained hands.

Clerics represent people of genuine faith whose prayers are actually answered. In a Lovecraftian setting, the source of that power becomes uncertain. Some clerics serve benevolent forces through traditional religions. Others worship cosmic horrors masquerading as gods or demons wearing divine masks. The Life Domain provides healing and protection that becomes precious when magic is rare and dangerous.

Rogues fit virtually any setting, and the 1920s offer plenty of opportunities for their skills. Cat burglars, confidence artists, bootleggers and professional criminals all thrive in an era of Prohibition and organised crime. The Assassin and Thief subclasses translate directly, representing everyone from trench raiders who learned to kill silently in the war to acquisition specialists who retrieve items better left undisturbed.

Monks bring a touch of Eastern mysticism that, while not strictly Lovecraftian, fits the period and expands player options. The 1920s saw growing Western interest in Asian philosophy and martial arts. A monk might be an immigrant preserving ancient traditions, a veteran who studied abroad or a scholar who discovered that certain meditation practices unlock capabilities beyond ordinary human limits.

The Detective Class

Investigative horror needs investigators. The new Detective class fills this role with two distinct subclasses that reflect different approaches to the unknown.

The Private Eye is the hard-boiled detective of pulp fiction: tough, pragmatic and willing to get their hands dirty. This subclass pays homage to the noir tradition that emerged alongside Lovecraft's cosmic horror, providing combat capability and resilience while remaining rooted in investigation. Private Eyes gain fighting styles, extra attacks and the ability to shrug off damage that would drop less experienced characters.

The Paranormal Investigator takes a different path. Whether a psychic medium, a skeptic forced to confront the impossible or someone forever changed by an encounter with the unknown, this subclass develops abilities that defy rational explanation. Sixth sense, spirit sight and psychic impressions make the Paranormal Investigator invaluable when facing threats that leave no physical evidence.

Both subclasses share core Detective features: keen observation, forensic analysis, the ability to read people and follow hunches, and investigative techniques that let players piece together mysteries the way Lovecraft's protagonists do. The Detective class ensures that R'lyeh Rising supports the investigation that drives cosmic horror without forcing every character into the same mold.

Adventures Ready to Run

R'lyeh Rising includes "The Rats in the Walls," a complete adventure adapted from Lovecraft's classic tale. Players investigate Exham Priory, where the last heir of a cursed family has uncovered something ancient and hungry beneath the estate. It's designed to introduce the setting's tone while demonstrating how investigation, horror, and combat work together.

But that's just the beginning. I’ve already developed additional modules adapting Lovecraft's stories: 

"The Temple" takes players aboard a doomed submarine discovering impossible ruins beneath the Atlantic. 

"Herbert West: Reanimator" plunges players into the consequences of science without ethics. 

"At the Mountains of Madness" sends an expedition to Antarctica where they'll uncover truths about Earth's prehuman past. 

"Dagon" (really a mashup of "The Shadow Over Innsmouth" and the original "Dagon") confronts players with the full horror of the Deep Ones' centuries-long corruption of a New England town.

These modules are available as digital and physical additions to the core campaign setting through the Kickstarter. Each was developed alongside the setting and extensively playtested as part of its creation.

The Process: Human Creativity and AI

I’ve been transparent throughout the development of R’lyeh Rising that AI assisted in its creation. Every rule was conceived and playtested by people. Creatures were balanced through actual play, and all lore was reviewed against Lovecraft’s original texts.

Dungeon Matters develops games and explores ways AI can support game creation, group play and AI-enabled solo adventures. For R’lyeh Rising, AI assisted with development, testing and imagery, while the game’s creative direction, design decisions, review and playtesting remained human-led.

I will always be open about how AI is used in our work. With that in mind, here is a closer look at the image-development process behind R’lyeh Rising.

Imagery

The imagery for R’lyeh Rising was created with the help of AI, but the process involved much more than entering a prompt and copying the result into the book. Developing a consistent illustration style, recurring characters and a visual narrative required experimentation, revision, and extensive retouching and editing.

For many illustrations, I generated multiple versions, selected the strongest elements from each and brought them into Photoshop. From there, I composited images, adjusted poses and proportions, rebuilt details, retouched faces, hands and clothing, corrected backgrounds and added manual illustration where needed. Some finished pieces contain elements drawn from several generations, combined and reworked into a single image.

I didn’t want the artwork to feel like a collection of unrelated AI images. The goal was to create a coherent visual identity and use the illustrations to tell a story that develops alongside the campaign setting. Characters recur, locations change and the imagery becomes darker and stranger as the Mythos emerges. AI was one part of that process, but the final results depended on art direction, iteration and a considerable amount of manual craft.

Example of image compositing and retouching for the Cover Illustration.

Original Base Layer

Final Image

Telling a Visual Story with AI

One of the greatest challenges in working with AI-generated imagery is maintaining consistency across multiple illustrations. A single strong image is relatively easy to produce. Creating the same character repeatedly, from different angles, in different poses and across a sequence of scenes is much more difficult.

For R’lyeh Rising, I wanted the artwork to follow a recurring cast drawn from characters used during playtesting. I began by deciding who would appear throughout the book and what role each character would play in the visual story. Many were partly inspired by people I have played RPGs with, their characters or memorable moments from the R’lyeh Rising campaign.

The cast includes a professor who is also a warlock, a firearms specialist using the setting’s Gunner subclass, two Umbral circus performers, a Deep One hybrid ship captain, a hard-boiled detective and several supporting characters who make occasional appearances.

Developing the Illustration Style

Before developing those characters in detail, I needed to define the visual language of the book.

I experimented with several illustration styles across multiple AI image-generation platforms. I eventually settled on a charcoal-and-pencil concept-art style inspired by 1930s pulp fiction. The linework and heavy shadows suited the period while giving the illustrations an unsettling quality that felt appropriate for cosmic horror.

To develop that style, I began with a public-domain photograph. Before using it, I confirmed that both the original image and the specific digital file were available for reuse. I selected a First World War photograph from 1916, sourced through Wikimedia Commons, as the reference image.

I then used the photograph as a compositional reference and generated illustrated interpretations through several AI image platforms. I tested different prompts, adjusted the visual instructions and compared the results until I found a style that worked for the image and could also be reproduced with reasonable consistency across other scenes.

The final result was not based on a single prompt or generation. It emerged through repeated testing, comparison and refinement until the charcoal-and-pencil treatment felt distinct, historically appropriate and flexible enough to use throughout the book.

Original Source Image

Final Illustration Style: 1930s Pulp-Fiction-Inspired Charcoal and Pencil

Once I had a prompt that could reproduce that look with reasonable consistency, I developed a second version that added watercolour-inspired colour. This gave me two related visual styles. The black-and-white style worked well for frequent illustrations and generally required less retouching. The colour style was reserved for major pieces that justified additional generations, compositing, repainting and Photoshop work.

This was less about discovering a single perfect prompt than building a repeatable process. I tested variations and image generation tools, and gradually refined the language until the images began to feel as though they belonged in the same book.

Pulp-Fiction Inspired Charcoal and Pencil Style

Character Development

Each major character began with a written description and a series of exploratory prompts. I refined the results through additional prompting in both ChatGPT and Midjourney, then used Photoshop to adjust details.

Once a character’s overall appearance was established, I brought the image back into ChatGPT and generated character studies showing different poses, expressions and viewing angles. These studies were not treated as finished artwork. I downloaded them, selected the most useful versions and refined them further in Photoshop until the images felt right.

For each major character, I eventually assembled three core references: a finished posed illustration, a character study and a detailed written description. I uploaded those references into a dedicated ChatGPT conversation used specifically for character imagery and assigned the character a simple title such as “Warlock Professor” or “Umbral Acrobat.”

I repeated this process for the principal characters and creatures appearing throughout the setting. Once the references were established, I could request a scene using those titles rather than rebuilding every description from the beginning. The reference images, written descriptions and consistent illustration prompt worked together to produce characters who remained recognisable from one scene to the next.

The process was never completely automatic. Consistency came from generating multiple versions and combining AI with manual compositing and retouching.

Character Study Examples

Warlock Professor

Carnival Umbral Strong Woman

Umbral Acrobat

Visual Storytelling

The final step was using those consistent characters to create a visual progression across the book. Rather than treating each illustration as an isolated decoration, I planned images as moments within a larger story. This allowed the artwork to show change, escalation and consequence without requiring additional exposition.

Join the Kickstarter

R'lyeh Rising launches on Kickstarter Tuesday, July 15th. The campaign will offer the complete setting book, the adventure modules, digital and print versions.

If you've wanted your players to feel genuine dread rather than confident heroism—if you've wanted to explore Arkham, Innsmouth, and realms beyond human comprehension with a rule system that’s anything but forbidden knowledge, play R’lyeh Rising.

Back R'lyeh Rising on Kickstarter →

R'lyeh Rising is developed by Dungeon Matters. Follow us for updates as we approach launch, and join our community to connect.