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Sandy Petersen's Cthulhu Mythos is a series of guides that adapt the Cthulhu Mythos to High Fantasy Tabletop RPGs, developed by Sandy Petersen, the original designer of Call of Cthulhu, and published by Petersen Games. It includes new rules for insanity and dreams, new spells and rituals, monsters and items related to the Mythos, details on cults of the Great Old Ones and Outer Gods, new player races created by H. P. Lovecraft—Dreamlands Cats, Mythos Ghouls, Gnorri and Zoogs—including new character options for them, and a new kind of stat block to represent the Great Old Ones and Outer Gods. These Elder Influences, as they are called, are about halfway between a monster and an environmental effect, and they represent the way that Mythos deities can impact the world by their mere presence. Reality itself gets weird when these things show up, and many of them are a serious challenge for even high-level parties.
Sandy Petersen's Cthulhu Mythos was released for Pathfinder 1st Edition in 2018, for Dungeons & Dragons 5th Edition in 2019, for The Dark Eye in 2020, and for Pathfinder 2nd Edition in 2021. In addition to the core book, a series of full campaigns, which span from level 1 to level 14 or 15, are available for the D&D 5E version, and can easily be introduced into any fantasy setting.
NOTE: Tropes relating to Pathfinder, Dungeons & Dragons or the Cthulhu Mythos as a whole should go on those pages.
This game provides examples of:
- Achilles' Heel: Yog-Sothoth's manifestation depends on a physical or magical portal. If this portal is destroyed, Yog-Sothoth's influence immediately ends. Sandy Petersen notes that the best way to deal with Yog-Sothoth is not to battle him directly, but to destroy the arcane passageway giving him access to our realm. Once it closes, he must immediately depart.
- Adaptation Expansion: The Great Old One Gobogeg was expanded by Sandy Petersen from the 'moon-ladder' and the 'elder Pharos' in the awful vision which Danforth received at the end of At the Mountains of Madness:He has on rare occasions whispered disjointed and irresponsible things about "the black pit", "the carven rim", "the proto-shoggoths", "the windowless solids with five dimensions", "the nameless cylinder", "the elder Pharos", "Yog-Sothoth", "the primal white jelly", "the colour out of space", "the wings", "the eyes in darkness", "the moon-ladder", "the original, the eternal, the undying", and other bizarre conceptions; but when he is fully himself he repudiates all this and attributes it to his curious and macabre reading of earlier years.
- Adaptation Name Change: The Bloody Tongue, a mask of Nyarlathotep originally introduced in Call of Cthulhu, is renamed The Howler in Sandy Petersen's Cthulhu Mythos.
- Ascended Extra: Zoogs, gnorri and Dreamlands cats were all very minor entities in the overarching Mythos, being only mentioned in passing in the Dreamlands cycle. Here, they get promoted as central playable Mythosian races.
- Amazing Technicolor Population: Gnorri are divided into a number of different subraces, which can be distinguished entirely by their coloration. The most commonly encountered subrace, which occupies the high water column and continental shelf, is blue. Gnorri from the deepest parts of the ocean are black, red gnorri occupy the "middle zone" between the black and blue gnorri, gnorri from arctic environments are white, and gnorri that have migrated to brackish and freshwater environments, such as swamps and rivers, are mottled in color.
- Bizarre Alien Biology: Iconic to Mythosian works as a whole, but particularly notable examples are the Gnorri and the Cats from Saturn, who have the ability to extrude and absorb limbs from their bodies, although they can only produce so many before they weaken themselves as a whole.
- Bizarre Alien Limbs: Both Cats from Saturn and Gnorri can extrude limbs by manipulating their biomass. Gnorri can produce up to 4 limbs, but the strain of doing so and lack of benefit means that they usually only keep out three at a time. Cats from Saturn can have up to seven legs, but any more than five stretches their bodies increasingly thin, giving them increasingly pronounced Fragile Speedster effects.
- Bizarre Alien Reproduction:
- Gnorri have three sexes; males, females, and gender-neutral "builders", who don't contribute biologically to reproducing but have intuitive earth elementalism, which they use to create sheltered bays that the males and females use for reproducing. Females expel clouds of eggs roughly once or twice per month, with males instinctively reacting to the presence of eggs by discharging sperm in a manner likened to sweating. The larvae exist as microscopic plankton for the first 2-3 months of their life (which results in the vast majority being eaten) before growing into a voracious sub-adult, which resembles a human-sized eel; this stage lasts for three years before they mature into a fully developed and functional adult.
- Downplayed with the zoogs; they're marsupials, which exist in real life, but male zoogs also have pouches and can thusly use these to help rear their infants.
- Black Speech: Attempts to understand more than a primitive smattering of Aklo can result in madness, mania, or even death. Generally, this is represented in the dread and insanity rules as a moderate or major disturbing discovery.
- Canon Immigrant: Following a perennial tradition of the Cthulhu Mythos, the series introduces or expands upon a few original creations of Sandy Petersen and Kickstarter backers, including the Great Old One Gobogeg, the Tcho-Tcho Okkator, the abyssal custodian, the eremite symbiont, the yothan, and the Watcher of the Green Pyramid.
- Cognizant Limbs:
- Ghatanothoa's rootlings, which resemble pseudopods, are treated as individual creatures, and all must be destroyed (alongside the nucleus) to end Ghatanothoa's influence.
- Ubbo-Sathla's nucleus contains two, four or six Tablets of the Gods, depending on its stage of influence, and relies on the Tablets of the Gods for structure. They can be attacked, and destroying them causes the entity to fall apart, ending its influence.
- Composite Character: All kinds of humans that become animalistic and vile under outside influences, from Arthur Jermyn to the inbred, cannibalistic Martense family in The Lurking Fear and the monstrous quadrupeds in The Rats in the Walls, are all treated as variants of Mythos satyrs and have identical stats.
- Duel Boss: In order to defeat Hastur's influence, each observer must defeat their own observation of Hastur that appears before them. One cannot interact with a version of Hastur observed by someone else, but can aid someone else indirectly against their observed Hastur (such as via healing or removing debilitating effects). Once all creatures in the area of influence defeat Hastur, the influence ends, provided at least one observer remains standing.
- Evil Virtues: Tcho-Tcho are usually chaotic evil, but when it comes to their own communities and families they're incredibly selfless. Tcho-Tcho who turn out to be Token Heroic Orcs are even still accepted by their families unless things escalate really far.
- Fantastic Caste System: Zoog society revolves around a complex hierarchy which is determined in primary by how many generations of ancestry one can claim occupied the same set of territory, and in part by both the generational status of their parents and by which of their parents' pouches they were gestated in.
- Fighting a Shadow: An important thing to remember is that Elder Influences do not act as if they are monsters. Although an Outer God or Great Old One's local influence has hit points and might in some sense attack, these represent but a minor aspect of the entity in most cases, rather than the whole. All influences can be defeated by certain conditions or actions, but this does not permanently defeat the entity itself, whose influence can simply manifest again as the conditions allow.
- Fusion Dance: This version of Fthaggua and Cthugha does not exist as individuals. Instead, they are gestalt entities, formed of many fire vampires (a few hundred for Fthaggua, thousands or millions for Cthugha) joined into a single, shared mind and body.
- Hostile Weather:
- The area within Cthulhu's stage 4 influence is wracked with powerful storms, earthquakes and tsunamis as appropriate.
- Ithaqua's area of influence is constantly wracked by an immense arctic squall.
- In Case You Forgot Who Wrote It: This sourcebook is officially known as Sandy Petersen's Cthulhu Mythos.
- Make My Monster Grow: Azathoth's influence is immobile, but once it is enraged, it begins to rapidly grow in size.
- Mask of Power: The influence of the King in Yellow is inexorably linked to the Pallid Mask. When an appropriate host finds an active Yellow Sign, the Pallid Mask compels it to touch the sign, then manifests on the host's face, turning it into the King in Yellow.
- Mechanically Unusual Fighter: For the most part, Elder Influences function more like hazards or environmental dangers than creatures. Simple influences attached to proxies function like typical combat encounters with those proxies, while simple influences without proxies function like environmental conditions. Complex influences function much like complex hazards.
- One-Winged Angel: Some Elder Influences have multiple stages of influence to model their growing power. For every stage, the Elder Influence's CR (or level in the Pathfinder 2nd Edition version) grows by 3. For example, Cthulhu is a CR 21 Elder Influence at influence stage 1, but increases to CR 24 at stage two, CR 27 at stage 3, and finally to CR 30 at stage 4. Elder Influences escalate (advance to a higher stage) when certain conditions are met.
- Rat Men: Zoogs have never had a clear visual design, and in Petersen's own artbooks have been depicted as both possum-like creaturesnote and giant ratsnote , with the only consistent design being the plethora of tentacles around their mouths. In this book, they are presented as small humanoid rats that can operate with equal ease as bipeds and quadrupeds.
- Ritual Magic: At stages 1 through 3 of Shub-Niggurath's influence, sacrificing living animals of the right type and via the proper ritual can cause her to extend more of her presence (escalating her influence) or retreat more quickly (de-escalating it).
- Rules Lawyer: The most important thing to keep in mind when dealing with zoogs is that their culture emphasizes the importance of the letter of the law, not its spirit. Whilst they readily make contracts, they are also perfectly happy to exploit any perceived loopholes in those contracts, and in fact are implied to suffer an almost Stupid Evil level compulsion to exploit loopholes, which has backfired on them more than once.
- Self-Duplication: Once per day, when Nyogtha kills a creature by reducing it to powder, a new avatar of Nyogtha manifests within the its area of influence at any point chosen by Nyogtha.
- Sizeshifter: Hastur functions differently at different distances, as the natural laws of perspective would suggest. His influence grows more potent as he descends from the sky where he has been called, and he appears to grow larger as he does so, but the relationship is disconcertingly counter-intuitive. His apparent size for anyone who has observed him during this influence is only initially based on the distance between him and the observer. When someone inside the influence notices Hastur, his perverse geometry creates a connection to that observer. For as long as Hastur's influence lasts, his apparent size is thereafter based upon his shrinking distance from the area of influence. Even if the observer moves away, Hastur appears to be roughly the same size. Indeed, he continues to grow larger as he approaches his destination, even if the observer has since moved farther away.
- Stationary Boss: Certain Elder Influences have a speed of 0 and cannot move, usually (but not always) because their nucleus is an inanimate object.
- Super Soldiers: Tcho-Tcho Okkator are selected and trained to be enforcers. Tcho-Tcho biomagic and alchemy enable truly terrifying modification of the Okkator. They can grow venomous fangs or extra limbs, or develop the ability to spit acidic webs out of their mouths.
- Timed Mission: When Quachil Uttaus' influence settles over a region, its nucleus typically manifests in a well-hidden area. If a short magical incantation is performed, Quachil Uttaus immediately relocates to the location where that ritual was spoken, where its presence could be attacked directly. Though this incantation is simple to recite once learned, the precise words that must be spoken are well hidden and often change each time Quachil Uttaus is summoned. The effort required to learn the incantation should consist of a significant adventure in and of itself, pitting the PCs in a race against time as Quachil Uttaus' influence continues to slowly degrade the region it is hidden within.
- Touched by Vorlons: The Tcho-Tcho and the Leng are basically the same thing to humans as drow are to elves, except the reason they're different (psychologically as well as physically) is the influence of the Elder gods.
