Cthulu's world by QuantumBranching on DA.
"Two hundred years ago, the Stars Were Right. R’lyeh arose, and the most psychic 20% of mankind either went mad from fear, killed themselves or went gleefully psychotic. The seas arose as the new land mass displaced uncountable tons of water (not all at once, admittedly) and billions of Deep Ones flooded ashore. Vast abominations emerged zombie-like from the Earth, rose from the seas, or ripped their way out of sinister mounds and creepy mountains. Things that should have been long dead emerged from countless hidden tunnels. Darkness fell upon the Earth.
Nuclear weapons could at most annoy Great Cthulhu himself and destroy some of his stuff, but a sufficient number of them could in theory wipe out most of humanity and deprive him of subjects, as well as doing a serious number on his oceanic minions and his Spawn, as well as some of the lesser Abominations. A rump US and Russia managed to negotiate a tenuous armistice on the basis of “you don’t eat our brains, we don’t blow up the world to spite you.”
Two centuries passed. The continent underlying R’lyeh arose further, and the seas penetrated deeper inland. The spawn of the Great Old Ones multiplied and built monstrous cities across the face of the earth. The wild shoggoths of the sub-Antarctic were taught obedience with the aid of the Primal Abomination which lurked atop the highest peaks of Antarctica. Some disputes with distantly related horrors deep within the earth were resolved with the death of only a few tens of millions of humans, mostly from famine from the ash and dust released into the atmosphere, although some through submersion in hot magma. (Humans are still unsure if this was a serious challenge to their rule, or the equivalent of someone banging on the roof with a broom because the kids are playing their damned music too loud). The ancient enemies of the Great Race of Yith emerged from underground and claimed Australia as their own, inviolate territory.
Now, while in some places the Great Old Ones take a direct hand in the management of their human subjects (and in some places not so much manage them as graze on them), in most areas they rule the world indirectly, through the heirs of the Cthulhu Cult and opportunistic evil bastards and mages, some (relatively) pure humans and others of mixed human-Deep One blood, an all-powerful priesthood whose rule as “shepherds” of the human flock includes slaughterhouse delivery and whose rule is backed by divine retribution far more swift and sure than any previous variety . Mass human sacrifice is pretty universal, although the details of how the various human states are ruled are otherwise of no great concern to Cthulhu and the other Great Old Ones which are his “family” and friends. Bloodlust, violence and weird sex are the social norm: close influence of the Great Old Ones may lead to power, but it leads to either crippling madness or the madness of utter inhumanity in the long run, and as for those too psychically insensitive or strong to be driven to mindless worship by the general psychic background, they are reduced to a state of foggy placidity by various foul magics, such as television. (Yes, there is television in the Cultist lands. There is a lot of television. Trying to tune in to Cultist TV is a death sentence crime in the US and Russia).
Besides TV, technology is an odd mix, mass slave labor and 20th century hardware, cities more monumental than New York but no computers, the occasional odd bit of unearthly super-science and the use of weird forces best called “magic.” The Great Old Ones prefer their subjects and soul-munchies ignorant: research, scientific or otherwise, is not encouraged. There are forces Man Was Not Meant to Know, given that some of them could actually inconvenience the Great Old Ones. Such as summoning the Outer Gods.
Living standards vary wildly. Some Cultists try to improve things, seeing their own wealth and power being increased by More Pie, or their predecessors having looted the place so thoroughly that there is hardly any Pie left. Others simply squeeze the rest of the population, even unto death – and beyond, given the value of zombie labor. Some areas, once you discount for the existential terror, have living standards not that far behind the first world of OTL, at least for those who aren’t actual slaves: others are the worst of third-world squalor interrupted by the occasional monument city, while still others would be counted as failed states OTL, except the local cultists like it that way. Much depends on whether there is a local Great Old One interested in whether the temples tend to stay up or fall down, and whether the Shoggoths run on time. The servants of the Great Old Ones which please them get to indulge every foul pleasure that can be conceived: those who annoy them perish in ways horrible beyond conception – if they’re lucky enough to die.
Cthulhu is worshipped everywhere, but it’s sort of a Zeus and other Gods situation: there are local cults which have other “favorite” deities, or have a Great Old One in the neighborhood which might be stirred into unpleasant action at not getting enough sacrifices. Generally, even small towns/work camps/mass torture facilities have multiple shrines.
The spawn of the Great Old Ones and associated horrors – Cthulhuoids (like the big C, but smaller: think classic King Kong vs. Godzilla), shoggoths, night-gaunts, zombies and litches and giants made of many human bodies crudely smushed together, Hounds of Tindalos and hideous hybrids great and small, walking trees and crawling boulders, etc. – inhabit and build the monster cities of dubious geometry that disfigure the face of the Earth, with some of the detail work being done by disposable human labor. What everyday life (or undeath) is like in these cities is unknown – for all but the eldest and most sanity-deprived  of the cultists, they work like a roach motel unto Blattidae, humans check in but do not check out. There is often but not always a full Great Old One in residence in the central temple complex, sometimes more than one; Cthulhu generally stays in R’lyeh, although he occasionally visits his human subjects for a little Al Fresco dining (Yum yum!) and to remind his human subjects Who’s the God? Some areas are dominated by a particular city and its chief Abomination, or by two or more Abominations working together, while others get only distant and spotty monitoring by local GOOs.
It’s not just Cthulhu: there are a number of Great Old One cities on the Dark Continent, many of the horrors being apparently uninterested in close interaction with humanity. Details are lacking; not even the heads of the Cthulhu cult have been allowed onto the Dark Continent and into R’lyeh itself. The Dark Continent itself is disturbing in various ways, from the physics-defying Black Lake to the lack of any mechanism to explain its rapid rise from the sea floor, and then there’s the vegetation: initially bare rock and sea-floor slime and sediment when it arose, has in the last two centuries developed an increasingly dense covering of weird and disturbing vegetation, black and purple, fungoid and mobile, quivering, pulsing, and shooting out clouds of spores, where it is not spiky, geometrical and impossibly large. Ecologists are greatly worried about what might happen if it spreads to the other continents.
Antarctica is still most frozen, the sea level having risen due to the emergence of R’yleh rather than global warming, but it _is_ melting as the world continues to warm: Greenland is partially submerged, and the sea level continues to rise. There are no Great Old One cities in Antarctica, which is the sole possession of the Primal Abomination and its Shoggoth servants, which It lets the other Old Ones borrow often enough as slave labor. Although such an idea is not even to be thought of in the Cultist lands, some in the US and Russia suspect even Cthulhu is a bit afraid of the Abomination, which has been on Earth far longer than the Old Ones, and is rumored to have been atop the mountain when it first arose from Earth’s primordial, lifeless sea.
Other things prefer to live under the Earth or in the seas, or in dubious spaces above and beyond the Earth. Vast worm-things burrow through earth and stone, the Deep Ones frolic on coastlines and islands with no fear of humans with machine guns, dubious masked and hooded figures come into the cities of the Cultists to haggle and trade for various things, most often human slaves.
The huge polygonal region surrounded by 2000-foot walls remains wrapped in impenetrable darkness, while the Atlantic Vortex is quite visible but equally unexplainable: and these are only two of seven equally vast and obscure projects the Great Old Ones and their servants have created in the last few decades.
These developments worry the Democratic Human Republic of America, which has made determination of their nature a top priority. After a long period of instability and various withdrawals in the face of irresistible pressure, the Democratic Human Republic of America has managed to achieve some stability: there hasn’t been a pro-Cthulhu revolt above pants-bomber levels of competence in decades. Given existential despair at the continued non-appearance of Jesus & Co., and the frequency with which the religious-minded were befooled by the Great Old Ones, the theocratic rule of the Republic’s early decades failed to sustain itself, and has been replaced by a ruthless scientific atheism as the creed of the Republic.  Although the Government’s refusal to call a spade a spade or Black Magic Black Magic is perhaps a weakness, the insistence on the non-demonic nature of the Great Old Ones and the eventual scientific explicability of all their mysterious powers is something of a spine-bracer. 
Powered by fusion reactors, bristling with atomic weapons, and recycling every scrap (there’s not exactly much trade with the rest of the world, although a lot of smuggling), the Republic is more democratic than it used to be, but is still a fearfully regimented society, in which any deviation from the Rules for survival is punished ruthlessly. Psychic sensitivity is tested from infancy, and all “susceptibles” are required to wear protective head-gear (aluminum-foil lined and marked with certain ancient symbols) whenever outside, and susceptible children are supplied with helmets they can’t take off. (All buildings are heavily psychically shielded as a matter of course). Constant secret monitoring for signs of Alien Influence is an accepted norm, as are physical and blood tests for signs of Monsterism. Privacy is a nearly dead concept, although on the positive side many venial sins of our era no longer cause much excitement.
There has been a return to space, although made risky by the fact that most of the Great Old Ones and many of their minions can move through space with little difficulty (How Not to be Seen is an important part of space science: low-flying spy satellites in particular tend to cease working rather abruptly, especially when passing over R’lyeh). The Moon is developing strange surface markings that will bear careful examination: there are plans for a manned trip to Mars, which at least still looks normal.
Robotics is quite advanced, although technology has reached a bit of a roadblock now that AI has reached the level of development to be able to go insane.
The Republic is not entirely alone in a world of Cthlhu-worshippers: radio communications remain open with the New Russian Empire, and a few isolated human holdout states in the western Amazon basin and other places. Other forms of communication are trickier: it takes a brave and highly skilled individual to travel undercover through the slave nations (and if you catch the attention of something which can read minds, you’re screwed), boats with engines and subs are too easily heard by the Deep Ones (and sailboats are generally not big enough to be safe from some of the new Horrors of the Deep), and planes are also rather visible. Taking a detour through other dimensions always carries severe risk of running into something horrible or just getting permanently lost – safe routes are still being sought for at a terrible cost in lives. Besides communications difficulties, relations with the Empire are inherently a bit shaky, given its far more open embrace of Black Magic and Abominations in their pursuit of survival. (There is considerable suspicion that the Fifth Putin, which has ruled for over fifty years, is no longer technically human).
There is also the Sultanate of Dust in the Arabian Desert: its inhabitants no longer look very human even on satellite photos, and they appear to have managed to remain independent through some sort of terrible pact with the even more powerful Outer Gods. (One of the more creatively decadent Saudi princes discovered that his multi-million dollar collection of blasphemous books and scrolls was actually useful while the world was coming to an end). Communications are understandably limited.
(British scholars tried to pull the same trick with the rich variety of mystical texts the British empire had plundered from half the world, but perhaps being in a hurry due to madmen and Deep Ones already overrunning most of England, they made an unfortunate error and summoned up something uninterested in deals, something so ghastly that the Great Old Ones were forced to take steps to confine it to Britain east of Wales. Very few Brits made it out alive, and even a few minor Great Old Ones just freshly awakened from slumbering under Britain were forced to leave for the continent with their metaphorical tails between their legs).
Other “free humans”, refugees from the Archonate or the Golden Empire of the cult masters, squat on the territory of the Mi-Go: the chance of being abducted and experimented on or even end up as a brain in a jar are probably higher than of becoming a snack or sacrifice back home, but at least the Mi-Go have no interest in governing humans or eating them, and as long as they stay away from the Mi-Go’s mining operations, they are free to pursue their primitive agricultural lifestyle unmolested. There have been complaints forwarded to the Great Old Ones, but the Mi-Go do have a solid short-term (less than a million years) lease.
Others inhabit Australia, which is fairly safe, at least for now, from the Great Old Ones, although you do have to worry about the Flying Polyps deciding they want to build some incomprehensible structure where your house is and blowing it away with a super-tornado. (Also, don’t build radio transmitters of any power. The Polyps apparently find strong sources of electromagnetism annoying, and what annoys them, they destroy). Some dwell in Underground Ethiopia, a holy man having led the last of the free Ethiopians into a secret underground world beneath the mountains of Ethiopia. After defeating the white, blind cannibals, they have settled in and live a life of prayer, frugality, and too many mushroom dishes, but there have been mysterious disappearances in the deepest caves, and some fear their refuge is coming to an end. There are small, fanatical groups in the highlands of Borneo, in the semi-desert of NE Brazil, in the mountains of what used to be Afganistan, Tajikstan, and north Pakistan, killing those who fall prey to the mental radiations of the Great Old Ones, sleeping with the aid of various drugs when tinfoil hats aren’t available, surviving, alas, because the Great Old Ones just don’t care enough to make an effort to wipe them out.
Other potential allies include the Chthonic Old Ones and the Outer Gods, but so far the Republic’s government has put them very solidly in the “Friends like These” category. There are also contacts with some discontented mages and dark creatures within the boundaries of Great Old One territory, who find that the Return of the Great Old Ones has not been nearly as cool or rewarding as promised or find being immortal and masters of life and death over millions does not make up for having the ultimate Boss From Hell. Special Intelligence has a new and very promising contact in Archonate New England, codenamed “TOM.” Many lower-level cultists are also unhappy, feeling that the often highly stratified cultist regimes are a betrayal of their fairly egalitarian past (in other words, they aren’t getting nearly as many bennies as they expected). And then there are the Shoggoths of Antarctica: having freed themselves after millions of years of slavery to the Elder things, they are quite unhappy to find themselves under the (metaphorical) lash again: they are also highly resistant to mental compulsion, not afraid of any of the other horrors aside from the Great Old Ones themselves, and have found that the heads of cultists and Deep Ones both make a lovely crunching noise when squeezed.
The Great Old Ones are not particularly concerned about the independent human states, although they are a bit miffed by the Outer Gods putting a tentacle in their pie. Two centuries are barely a blink of an eye to the immortal Old Ones: and in very little time (quite likely less, even, than a single century), certain plans will come to fruition, and the minor annoyances will end.
 Well, swifter, anyway. The Great Old Ones are hardly at the beck and call of their priests and tend to only interfere when things really hit the fan. Usually the cultists have to make do with Nightgaunts, zombies, Deadites, worm-men, evil ghosts, Familiars, maybe a Shoggoth or two if they have some real pull.
 I don’t mean they drool a lot and play with their poop – they are functional, those who cease to be so get removed by their subordinates it’s just that the software running on their meat computers is no longer compatible at all with Human Mind 1.0
 Which in turn has led to unrest among the remaining hard-core believers, who continue to insist that this is just a lengthy Tribulation and Jesus will be arriving any day now. At least they’re not as bad as the ones who insist we have to take a “leap of faith” by launching all the nukes at the forces of evil, depending on Jesus for the final blow.
 This is combined, perhaps a touch hypocritically, with extreme caution in the study of any and all “unnatural” texts, with frequent sanity and mental hygiene checks and careful segmentation of, say, material to be translated into many small presumably non-sanity damaging chunks. That this is necessary is evidenced by what has happened before elsewhere: Israeli intelligence’s “World War Z-type” caution in following up even the most ridiculous of rumored dangers was not matched by caution in actually studying certain blasphemous texts, leading to the spread of cultists through the Israeli intelligence services like mold through a piece of bread."