Originally named the "Yog-Sothoth Cycle", this is an informal and, appropriately chaotic Shared Universe that squarely defines the darkest and edgiest of genres, cosmic horror.It was started unintentionally by HP Lovecraft and his circle of peers (informally called the 'Kalem Club') who belonged to the embryonic Fandom. At that stage less about Speculative Fiction, and more about writing short amateur "weird" stories for the 'pulp' magazines, at least for Lovecraft.Lovecraft had already incorporated small elements of Robert W Chambers' earlier The King in Yellow and the writings of Arthur Machen by way of Shout Outs. As time went on, Lovecraft and his friends began referring to his Eldritch Abominations and Tomes of Eldritch Lore in their writings, though usually not actual characters, and to share references made in his friends' stories or private letters. Mythopoeia defined the abstract, and original, cosmic setting.The actual term "Cthulhu Mythos" (depending on how you define it) post-dates Lovecraft's death, at which time H. P. Lovecraft's work got seized and expanded on by August Derleth. Lovecraft himself called his budding mythology "the Yog-Sothothery", because Yog-Sothoth features or is mentioned in many more stories than Cthulhu. Due to the Shared Universe's informal nature, several rather divisive conceptions of the Mythos have arisen, generally categorized as the Lovecraft purists' version; the version including the broad post-1930s expansions by later writers like August Derleth (who is a controversy unto himself) and Ramsey Campbell; and then there's the rigidly codified and de-mystified Tabletop RPG adaptations which crunch down Mind Screwdriver-style to produce orderly game rules from an inherently disorderly canon. Information from the latter has tended to proliferate across the Internet disproportionately, resulting in simple Google searches producing a majority of pages derived from the game and its various campaigns, which are not always labeled as such.HP Lovecraft has his own trope listing, so tropes here should be for tropes that are not specific to his work, or have been greatly expanded from his work. See also Cosmic Horror Story (for works which deal with Lovecraft's themes [and, optionally, make use of the Mythos) and Lovecraft Lite for works that take Lovecraft and Mythos less seriously.See also the Call of Cthulhu RPG.
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Sub-settings within the Cthulhu Mythos 'verse include:
Lovecraft Country: Lovecraft's own setting; Arkham, Dunwitch, Innsmouth, and Kingsport, and anywhere nearby that fits the imagery.
Campbell Country: Named after the work of Ramsey Campbell. Any Mythos-derived setting in Europe, most often England. The old castle from HPL's The Rats in the Walls and Ramsey Campbell's Severn Valley region are ideal examples.
The Dreamlands: Fantastical world created by people's dreams, focusing more on the surreal than terror.
Delta Green: The Mythos meets government conspiracies and black ops. This began as supplement for the Call of Cthulhu RPG game.
Hyperborea: Prehistorical Greenland before the ice age. Created by Clark Ashton Smith, it focuses more on the weirdness than horror.
A most recent addition, The Laundry Files series by Charles Stross is, by every indication, set firmly within Lovecraft's cosmology, as is its accompanying RPG. Much like in Delta Green, the monsters are real, the aliens and Deep Ones share the Earth with us, most governments have secret Occult Intelligence divisions, and The Stars Are Coming Right...
Alien Animals: Cats, at least in the Dreamlands, are intelligent, speak their own languages, and can leap across space. They're also at war with the Cats from Saturn.
Alien Geometries: One of the most notable examples being on the island of R'lyeh, in "The Call of Cthulhu".
Aliens Are Bastards: Just about everything not of this Earth is evil and/or horrifying. About the only exception are Elder Thing and the Great Race of Yith, who still do freaky things like body-swapping with humans so they can visit Earth, and politely mind-wiping the unfortunate human when they switch back.
Always Chaotic Evil: The majority of beings in the Mythos - the Deep Ones, Great Old Ones, the Tcho-tcho, the Insects from Shaggai - come off as this. Sometimes may be due to how alien these beings are. Sometimes not.
Ancient Astronauts: In addition to Lovecraft's Cthulhu, Elder Things, Mi-go, Yithians, and Flying Polyps, later writers added the Shan, Star Vampires and Yuggs.
Asshole Victim: A common occurrence. Though, the reader may feel pity towards these characters because of their horrific fates.
Author Appeal: Being a lover of cats, Lovecraft tended to treat felines rather fondly in his works. Friends and latter writers would actually add Lovecraft into his own Mythos because of his influence.
An unusual example is that Lovecraft himself became a figure in his own mythos and was written into several stories by other authors, either as an avatar or even more curiously as himself. In addition to this, the first Lovecraft short story collection The Outsider and Others, put together posthumously, was inserted into the mythos as one of the arcane tomes frequently referenced in the stories of other authors.
Robert Bloch killed off a character based on Lovecraft in "The Shambler from the Stars"; in response, Lovecraft killed off a character based on Bloch in "The Haunter of the Dark".
Latter, Bloch wrote a sequel which mentions both to the Expiesand to himself and Lovecraft. You would think that Bloch or Blake would have realized that they were carbon copies of each other down to having written almost identical stories. To top it off, the main character (Fiske) is also an avatar, as Bloch wrote under a pen name of "Tarleton Fiske".
Frank Belknap Long had another character based off Lovecraft in "The Space-Eaters".
The main character of Fritz Leiber's "Terror from the Depths" shares many similarities to Clark Ashton Smith.
Lovecraft himself had several avatars: Ward Phillips, Randolf Carter, and Abdul Alhazred (which was Lovecraft's childhood play name).
T.E.D. Klein's Black Man With A Horn has a character based off Frank Belknap Long.
Bizarre Alien Biology: Almost all the aliens in the mythos have this. There's fungus-crab-bat things, Crinoid/plant lifeforms, giant shape-shifting amoeba-like monsters, and giant telepathic squid-worms. And that's not getting to the Great Old Ones, who aren't even of normal "matter".
Blue and Orange Morality: An alternative take on the Mythos by some authors. Most of the beings in the Mythos are beyond good and evil, as we understand it. For example, Long presents the Hounds of Tindalos as "Foul" and "descended of angles". Humans are somewhat "Pure" and literally descended from curves. In "A Note on the Cthulhu Mythos", Derleth explains that the entities of the Mythos are "beyond mundane morality".
Continuity Nod: Given they are part of a Shared Universe, this is inevitable with many stories in varying degrees, ranging from very subtle such as an off-hand reference to Miskatonic University or the Necronomicon to more obvious ones like incorporating famous abominations, to outright referencing plot points from Lovecraft's stories, all of which can be put to great use depending on the nature of the tale.
Continuity Porn: Common from virtually the moment of Lovecraft's death onwards, though hardly mandatory.
If you consider many of the stories are narrated in first person and one of the bases of the Mythos is that humans actually know really little about the cosmos then we can conclude that the own character narrators doesn't know enough or have wrong information due to the vague nature of their sources about the Mythos, which of course is a nice excuse to keep creating new stuff.
Covers Always Lie: Lovecraft anthologies (especially the Del Ray ones) tend to have weird, surreal imagery that often doesn't have anything to do with anything in the stories. Though, it does communicate the atmosphere of the books well enough.
Cryptic Background Reference: Lovecraft did this often, and so did the Kalem Club, throwing out little bits and pieces of elder lore. It left fans wondering and wanting for more. Trying to piece them all together is part of the appeal of the Mythos.
Cults: The Mythos is filled with Old One worshipers with horrible rituals. They range from the Arkham Witch Coven, various madmen like the Whateleys, The Brotherhood of the Yellow Sign, and the English Temphill Cult to name a few.
Lovecraft's "The Dunwich Horror". Yog-Sothoth mates with a human woman and produces the offspring who will be known as Wilbur Whateley.
Michael Shea's "Fat Face". An "escort" seeks comfort from a large, seemingly kindly man. It's not a man, and it doesn't end well.
Played with Ramsey Campbell's "The Faces At Pine Dunes". The protagonist and his girlfriend is investigate his parents' strange behavior. His father is a Human/Eldritch Abomination hybrid, and so is the protagonist.
Dug Too Deep: Happens in a Lumley tale In which an oil drill ends up drilling into a sleeping Great Old One.
Earth Is the Center of the Universe: Played with - the primary reason that so many sealed evils are concentrated on Earth is precisely because they are not concentrated on Earth. There's just so damned many of them that Earth ends up having its fair share of octopoid elder gods as a matter of normal statistical distribution.
In The Dunwich Horror, young Whateley's diary states that the alien intelligences are interested in Earth as an element in their long-range plans. Organic life, on the other hand, is considered an obstruction, and their real plans can get started once they erase all life on Earth and take it out of three-dimensional space.
Gratuitous German: The Unausprechlichen Kulten. According to S T Joshi, a leading Lovecraft authority and who provides the annotations for Lovecraft's stories for the Penguin Classics editions, it's also wrong German. It should be "Die Unausprechlich Kulten". Odd, since Lovecraft spoke German. And it's still wrong: The correct form would be either "Unaussprechliche Kulte", "Die unaussprechlichen Kulte" or "Von Unaussprechlichen Kulten" (note the capitalization).
The Greatest Story Never Told: People journey into the depths of horror, sometimes preventing such nightmares from rising - and very few (if anyone) actually learn of the tale.
Homeworld Evacuation: The Insects From Shaggai (AKA Shan) in Ramsey Campbell's stories. When their home planet was destroyed by a Mythos abomination, some of them fled to a succession of other planets, finally ending up on Earth.
Yith itself is a dead planet, but the Great Race which evolved there fled to primordial Earth via mass Mind Swap.
Hostile Terraforming: Allies of the Mythos are trying to work towards "clearing off the Earth" for the Great Old Ones.
How We Got Here: Typically of the "See, this is why I must commit suicide before sundown..." variety.
Human Sacrifice: Whether it's being used in some unspeakable experiment/ritual or just being a snack for an Old One, it's very common.
I Have Many Names: All of the Great Old Ones and other incomprehensible beings have multiple aspects and/or names. Part of this is due to multiple attempts at spelling a alien word (Cthulhu, Ktulu, Clooloo, Q'thulu, Tulu, etc.) and partly just due to the use of epithet (Nuclear Chaos or The Daemon Sultan for Azathoth) in the case of The Scottish Trope where the true name is forbidden (even "Azathoth" is a pseudonym).
Kaiju: There are some really big monsters in the Mythos. Cthulhu is described as a walking "mountain". Most of his children are just are large. Not to mention Dagon, Zhar and Lligor, several of Nyarlathotep's masks... all big. And then there's Ghroth, a monster the size of a planet.
Living Bodysuit: Nyarlathotep, Hastur, Y'Golonac, and the Insects from Shaggai.
Loads and Loads of Races: HPL himself mentioned or sometimes showed a few dozen aliens and monsters, and subsequent authors and co-writers have expanded this greatly. That's not even getting into the godlike deity-aliens. Overall there are at least 30 intelligent races described.
Locked Out of the Loop: Upper-class Anglo-Saxons seem to be the only people on Earth unaware of the horrors going on.
The stories by Smith, Derleth, Lumley, and Howard in particular. Smith's tales focus on the weirdness than horror, and Howard's characters were simply Bad Ass enough to face cosmic horrors and fight them. Even Lovecraft had some lighter tales.
Naturally, the Mythos parodies and homages tend to be this as well.
Masquerade: One of the defining aspects of the Mythos, living in ignorance of the true horrors of reality.
Medium Awareness: Lovecraft encouraged the authors he corresponded with to use elements of his mythos in their stories, even if those stories were not part of the mythos itself. This emergence of common elements in seemingly unrelated works of literature created the impression that the mythos was actually real, thus leading to the fan theories that Lovecraft actually had encounters with eldritch entities. This culminated in a peculiar case when an infamous Moral Guardian by the name of Patricia Pulling included in a questionnaire submitted to police as a means of investigating people for possible occult affiliations, a question regarding whether or not the suspect had heard of and read the Necronomicon. This question, among various other things, led to her discrediting as a credible expert in the area of occult crime.
Moe Anthropomorphism: You can find most, if not all, of the mythos beings depicted as human girl. Cthylla is notable that, being called Cthulhu's daughter, it's much easier to search for her moe art than squid-like form. Nyarko San takes this to the next level, with Nyarlathotep, Cthugha, and Atlach-Nacha as cute moe girls.
Lovecraft's stories contain virtually no hanky-panky. His narrators are universally chaste. Females characters are almost consistently abominations in disguise. On the very rare occasions that sexual activity is implied, it is depicted negatively and guaranteed to result in inhuman hybrid demon spawn. The only even semi-notable exception is the unnamed mother of Charles Dexter Ward, who, while remaining a side character, deeply loves her son and falls ill out of concern for him; when her husband discovers what their son is really up to, he does everything he can to keep her from knowing just how awful things really are out of fear that he'd lose her forever, either mentally or altogether, from the shock.
There's the poor Lavinia Whateley in The Dunwich Horror, who goes over her head under the coaxing of her grandfather, and meets a grisly ending later one because she's not happy with the idea of destroying humanity. Most of the time women aren't so much evil as completely absent from Lovecraft's stories, since he had no idea how to write female characters. Even Asenath Waite was actually a man's spirit inhabiting the body of his daughter.
As in the space of a story (days, maybe weeks) the male heroes spend time among creatures like Innsmouth hybrids or man-eating degenerate beings from The Lurking Fear, it would be pretty horrible to imagine what they could do if they weren't chaste.
Averted with Smith and Campbell, where romance plays a role in some of their stories.
Non Indicative Name: Cthulhu only appears in one of the original Lovecraft stories, and his role beyond that one is fairly minimal. Though he is the most iconic character in the Mythos, he is definitely not the central figure—technically, "The Great Old Ones Mythos" would be a more accurate title.
Not So Safe Harbor: Not surprising considering how it's mostly set on New England, but Innsmouth is especially noteworthy. Also not surprising considering Lovecraft's phobia towards all things aquatic, thus marine and octopoid creatures as a consistent source of horror.
Occult Detective: Several characters attempt this, but often it doesn't end well. Titus Crow is a traditional example, while Teddy London is a private detective that worked on cases involving the Mythos.
The Old Gods: The Great Old Ones and the Outer Gods. Averted with the Elder Gods, who imprisoned the Great Old Ones.
Our Vampires Are Different: The fungoid creature in "The Shunned House" is Lovecraft's version of a vampire. It bears little resemblance to the undead humans of other works. Other authors added "Star Vampires" and "Fire Vampires", which are even less conventionally vampire-like.
Plant Aliens: Both the Mi-Go and the Elder Things are described as being fungoid.
Product Placement: Partly a Shout Out - in one of Derleth's stories, the characters acquire the anthology "The Outsider and Others" by H.P. Lovecraft for their investigation. "The Outsider" was the first book published by Arkham House, Derleth's and Donald Wandrei's company that was founded to help preserve Lovecraft's legacy. It was also a practical way to get the word out to the fans.
Poke In The Third Eye: In Long's The Hounds of Tindalos, the protagonist using mental time travel ends up going too far back and time, and attracting some really unwanted attention.
Popcultural Osmosis: Thanks to the innumerable pop culture references, people have learned about the Mythos from almost everything but the original stories.
Public Domain Character: Even when it was created. H.P. Lovecraft encouraged creative diversity in the original Cthulhu Circle, such that there was (and is) no single all-enjoining Canon, but rather what amounts to multiple authors' Ascended Fanon. In this sense, the Cthulhu Mythos more resembles an organic Mythology with numerous variations.
All over the place. Range from Derleth's ideas of morality or Smith's Greek pantheon-style genealogy (including such gems as Cthulhu being Hastur's half-brother), to Fan Wank trying to avert Science Marches On, like explaining various winged creatures like the Byakhee & Elder Things flying through space, originally ascribed to "ether", as biotechnological solar sails.
Lovecraft even did it to himself, such as placing the locales of some of his earlier short stories into the setting of the Dreamlands (you'll notice that 'The Cats of Ulthar', for instance, never mention being anywhere but in the real world, let alone a shared dream consciousness).
Sapient Cetaceans: One story in Tales from Innsmouth has the Dolphins as allies of the Deep Ones.
Sentient Cosmic Force: Yog-Sothoth for the Space-Time Continuum. The various Outer Gods could be interpreted to be this. For example, every nuclear reaction is Azathoth.
Science Fiction: Several of the various monsters are given scientific (or quasi-scientific) explanations and origins.
Scrapbook Story: Most famously, the original Call of Cthulhu story does this, and other writers have followed suit.
Sorting Algorithm of Evil: In essence, there are three tiers to the Mythos; at the bottom are the "mere" horrors — aliens, ghouls, mad sorcerers, etc. Above these, are the Great Old Ones, which are basically Physical Gods. And above these are the Outer Gods, which are to the Great Old Ones what the Great Old Ones are to the lesser races. To put it in perspective, Cthulhuworships the Outer Gods.
Spacetime Eater: Conversed between the narrator and a Lovecraft-expy in in Long's "The Space Eaters". They talk about Eldritch Abominations in general, with the Expy asking what would happen if they "eat their way to us through space!"
Spared by the Adaptation: The narrator in the 2005 silent film adaptation of The Call of Cthulhu. At the beginning of the original story refers to the "late" Francis Wayland Thurston. How he died is not revealed. The movie doesn't really hint at this at all.
Spotlight Stealing Title: Despite being the most well known and iconic character, Cthulhu himself has only a few small appearances in the stories and is more of a minor background character compared to Yog-Sothos, Dagon, and Nyarlathotep.
Starfish Aliens: All of HPL's aliens, and quite a few earth-dwelling creatures. Howard, for such an early writer, was good at ensuring his aliens were actually alien. And in the case of the Elder Things, one of the more sympathetic species, almost literalStarfish Aliens. Latter authors have followed suit.
Stuck In Their Shadow: In-universe example: The protagonist of Black Man With A Horn feels that his literary career was overshadowed by his friend, H.P. Lovecraft.
The Great Old Ones are some of the, er, oldest examples. All too often this aspect of them gets forgotten in favor of The Theme Park Version's literal gods.
In an inversion, in their introductory story, the Elder Things are presented as being men- that is, in comparison to the other aliens and horrors out there, the Elder Things built things, created a civilization, wrote, created, learned, taught. They built things and invented things. They're human compared to the nigh-godly Cthulhu Spawn and the horrific Shoggoth(s?).
Spawned several RPGs, including the Call Of Cthulhu RPG, Trail of Cthulhu (using the GUMSHOE system and focused in the 1930's), Arkham Horror and Yellow Dawn - The Age of Hastur RPG (set in a post-apocalyptic world).
The franchise also spawned Card Games, like the MythosCCG, the Call of Cthulhu Living Card Game, and even Munchkin Cthulhu.
In one short story a fan of Lovecraft in a world where the stories are in truth based on reality has surgery to allow her to pronounce R'lyehian correctly. This gives her an eldritch look, and when she actually practises the ability, it sort of causes the end of the world as a side effect.
The Cthonians dissolve in water. Justified: Considering the Cthonians are able to survive intense heat and pressures, can borrow underground, and have telepathic powers capable of controlling people's minds, the fact that Earth is mostly water may be the only reason why they haven't wiped humanity out.
Not a particularly exploitable weakness for the bigger ones though. Shudde-M'ell (the chief Chthonian) is described as a mile long, so immersing him in water would be ... pretty challenging.
Water isn't good for the Great Old Ones according to The Call of Cthulhu, either - it blocks their telepathic powers completely, trapping them to their lairs both physically and mentally, until R'lyeh rises again.
The Haunter in the Dark, one of Nyarlathotep's many forms, is extremely weak against light. Granted, it comes from a dimension where no visible light exists (and where it would presumably be invincible), and it can't be killed, only banished back to that dimension, but still, it's an Eldritch Abomination that can kept at bay with a flashlight! But you'd better hope your batteries last until you find something else... the Haunter can wait, it only needs to catch you once.
In Robert Bloch's story "The Shadow From the Steeple" (considered out of canon by some) it gets better: after a serious blunder by a university professor attempting to contain it, it takes over his body, therefore becoming almost unaffected by light, changes the man's field of expertise to theoretical physics, then joins the Manhattan project so we'll succeed in creating a weapon that could actually annihilate us. It's also an avatar of the god Nyarlathotep, The Crawling Chaos.
Call of Cthulhu itself offers one. You may be surprized that, despite being an ancient and unspeakably powerful entity able to drive to insanity with nary a glance, Cthulhu is just as vulnerable as anything else to being rammed with large objects.
Weird Tales: Many of Lovecraft and pals wrote for the magazine.
Who You Gonna Call?: Professor Shrewsbury, Inspector Legrasse, Titus Crow, The Wilmarth Foundation, Delta Green, and Teddy London.
Wolverine Publicity: Cthulhu only appears in one story, yet his name is used for the whole body of fiction. Justified in that Cthulhu or the events from The Call of Cthulhu is alluded to in other stories.
Word of Dante: Several common aspects of the Cthulhu Mythos (such as the good/evil dichotomy and the Necronomicon as a powerful Brown Note) come from Lovecraft's friend, August Derleth, rather than Lovecraft himself.
You Cannot Grasp the True Form: Contrary to popular belief, people can see the true form of many eldritch beings just fine. It's just sometimes the truth is just too much for the human mind. But there's a few that do play the trope straight. See the Character page for more detail.