The setting of the Pathfinder role-playing game is one of high fantasy with a touch of sci-fi and elements of horror. It's a Tolkien-like world with the Cthulhu Mythos incorporated into the setting, which have been part of the game from the beginning and play a major role in the setting. Originally Paizo published the Dungeons & Dragons games under contract to Wizards of the Coast from 2002-2006. When Wizards of the Coast decided not to renew the contract in 2007, Paizo continued on with Pathfinder under the Open Game License of the older version of D&D.
In the world of Pathfinder there are several dozen sentient species (normal, magical, aberrant, and demonic) living on the world of Golarion, some native and others not. Which includes dragons, krakens, giants, elves, dwarves, lizardmen, 100+ ft kaiju, demons, necromancers, vampires, zombies, naga, aboleth, serpentmen, merfolk, kitsune, spirits, elementals, and gods among many other races and species. Humans are one of the races and there are analogs of the various ancient and medieval human civilizations of Earth, and it's believed that the humans on Golarion are actually immigrants and/or refugees of another world (Earth). The greatest human civilization on Golarion is that of the empires on the continent of Tian Xia. Empires and kingdoms which are very similar to those of China, Japan, and Korea on Earth, and populated by both humans and non-human sentient species. The humans of Tian Xia have a physical appearance similar to the peoples of East Asia and Southeast Asia on Earth, and similar cultures.
The planet Golarion is part of a solar system with a number of inhabited worlds, each of which has multiple sentient species and the presence of magic, which altogether have over 100 sentient species of which some are from the system and others elsewhere (outside the solar system or from other dimensions). Through the use of magic (e.g. portals) some of these species have spread to neighboring planets within the system. The eldritch entities and species of the Cthulhu Mythos (known as the Elder Mythos) have a strong presence in the multiverse. The pantheon of the Elder Mythos are one of many pantheons and deities in the cosmos, with a good number in Golarion alone (some of which consist of human-like gods). While magic is a strong force in the universe, among the stars there are many spacefaring civilizations with high technology (interstellar starships, robots, artificial intelligence, directed energy weapons) and a few of those races have visited Golarion, some of them with an agenda and others due to their ship crashing on the world or as refugees. Resulting in alien artifacts (e.g. robots), mysterious alien ruins, wrecked alien ships, and isolated colonies of aliens. The worlds of these solar system also have portals to other dimensions inhabited by elemental beings, demonic races, etc., and with the right magic, one can travel to and from those dimensions.
Gods of the Mythos
In Golarion and its solar system, the gods of the Dark Tapestry (aka, the Elder Mythos) consist of two groups or pantheons. The Outer Gods are true gods, while the Great Old Ones are more akin to demigods, archdevils, or demon lords.
Great Old Ones
Great Old Ones respect the Outer Gods, and are often seen as their high priests or heralds. The Great Old Ones are nowhere near as mobile as the Outer Gods. In fact, many of them are imprisoned, waiting to be freed by a catastrophe. Which prophets across many worlds have predicted will result in the end times for their world and the Great Old Ones ascension into the Outer Gods' realms.
Mortals have a poor understanding of the Great Old Ones, who are believed to reside in the void between the stars, known as the Dominion of the Black or the Dark Tapestry. It's known that they are truly ancient beings, likely older than Golarion and it's human-like deities. Mortals that investigate these vast alien intelligences do so at their peril. Their followers and those who get their magical power by worshiping them are usually insane, and believe that that when they return the world will be unmade. Though some are limited in their ability to travel, they are able to influence the universe via mind control and dreams, and thereby even those whom are imprisoned can spread their influence to other worlds.
The Outer Gods in turn are a pantheon of incomprehensibly ancient and completely alien gods which exist outside of reality or the concepts of souls, faith, and quintessence. Beings who are so ancient, that according to scholars they exited before the Great Beyond and Pharasma, and therefore predated the concept of mortality. Most of the Outer Gods reside within the Material Plane, in the Dark Tapestry. Beings that are able to travel across the universe, and out of time and reality.
As vast, alien intelligences the Outer Gods perceive reality in such an inexplicable way, that they are unable to recognize what they do as being evil. Those who follow them or gain their magical power by worshiping them are usually insane, believing that when they return the world will end. Outer Gods in turn are usually unconcerned with mortals and do not desire or need their worship, and some of them aren't even understand the concept of faith.
Individual Entities of the Mythos
Known Great Old Ones
Yig = Great Old One of procreation, serpents, and cycles. He is considered to be the least evil of the Great Old Ones, but if he is hungry he will devour his worshipers. It's true form is that of a monstrous humanoid with scales and a snake's head and tail, standing 14 ft tall. He is worshiped by rural and nomadic communities. Unlike most of the Great Old Ones, Yig actually cares for those who worship him and will protect them. Though he is a snake god, the serpentfolk do not worship him. Yig is not known for holding grudges but is known for punishing those whom have harmed snakes or his clerics by turning them into snakes. Those who have earned his favour give birth to healthy babies and are rewarded with bountiful hunts and harvests.
Hastur = Great Old One of disorder, nihilism, and decadence. Whom is believed to be imprisoned in a distant world, within the alien city of Carcosa. Which is continously expanding and consuming other cities. Once it grows large enough, Carcosa is supposed to burst, at which point Hastur will become an Outer God. His cult rarely interacts with other cults, but orgiastic rituals are frequent between his cult and that of Shub-Niggurath (with whom it is believed he mated with). His cults usually pretend to be secret societies, with the initiates being unaware of their true agenda. Members have secret identities or pretend to worship other gods, and will send monsters or assassins to eliminate those who discover their secret.
Bokrug = Great Old One of storms, revenge, and water. For a Great Old One, it's relatively benign as it has no interest in destroying others and enjoys having worshipers. It will only use violence when it is angered or he has to avenge either his worshipers or himself. Bokrug's form is that of a giant spiny iguana with a long spiked tail. It is worshiped by swamp-dwelling races as well as humans. In exchange for blood sacrifices, it blesses its worshipers with wealth and fine weather. He doesn't care as to whether they sacrifice to him animals or sentient, but many cults sacrifice to him captives, criminals, and other undesirable humanoids.
Cthulhu = The greatest of the Great Old Ones, believed to be the Outer Gods' herald. Who lies sleeping in the dead city of R'lyeh, which is on a distant planet (Earth), where he is imprisoned by powerful magic and the Elder Sign. Despite this it is able to reach across the cosmos by touching mortal's sleeping minds. During which they dream of madness and afterwards are influenced by Cthulhu. He has form is somewhat amorphous, stands more than 100 ft tall, and resembles a corpulent humanoid with draconic wings and an octopoid face. Serving him are the Star-Spawn, who look like him though much smaller and weaker, and share his purpose. Worshiping him are those driven mad by Cthulhu in their dreams, various aquatic races, and those who live near the coasts.
Known Outer Gods
Nyarlathotep = Outer God of the Dark Tapestry, who is known to have taken thousands of forms. Each of these forms has its own unholy symbol, cult, and favoured weapon. Among the Outer Gods, only Nyarlathotep seems to find enjoyment in the manipulation of mortals and making them go mad. It's believed that he is preparing countless worlds for the Great Old Ones' apocalyptic return. The Dreamlands' strange gods are believed to be his creation. Nyarlathotep is their guardian and is said to serve as Azathoth's door ward. His cult's agenda is to bring chaos to their world, by directly or indirectly bringing about the deaths of reformers and kings. On Golarion, his most notorious forms are the Haunter of the Dark, the Faceless Sphinx, and the Black Pharoah.
Shub-Niggurath = Outer God that lives in the Dark Tapestry, who is mobile and therefore able to materialize on any world in the Material Plane. She appears as a huge blob of living tissue and mist with hooves, fanged mouths, eyes, tentacles, and other such deformities. Being a fertility goddess, it has mated with most deities of the Dark Tapestry, and her offspring are powerful. On Golarion, she has a strong following, with her cults usually located in rural areas (especially jungles, forests, or swamps). During the Age of Creation, she spawned millions of Neothelids into the planet's surface, most of who still worship her.
Yog-Sothoth = Outer God which lives within the Dark Tapestry. Which is believed to be the Dimension of Time's self-awared manifestation. It's true appearance is that of a conglomeration of glowing spheres, of various colors and sizes. Yog-Sothoth exists across all space and time, and the views of the Mythos Cults therefore a varied as the truth regarding Yog-Sothoth. Which include that Yog-Sothoth is the sapient manifestation of the Material Plane, that it is actually the Dark Tapestry itself, that it is some kind of transitive plane alongside the Material Plane, and so on. It is able to exist in multiple places and times at once, but either is not able to properly force itself upon the Material Plane or it has no reason to do so. It's cult is not organized and it's worshipers are often isolated and usually consists of insane spellcasters of arcane magic or are nearly all witches. He cares not for them, finding them useful only to mate with and produce his spawn or to be directly summoned into their world where it will then consume all lifeforms it can reach, corrupt the space and time it touches, and ravage the area.
Azathoth = Outer God, known as the Primal Chaos or Daemon Sultan. Which is an enigmatic, all-powerful entity which lies at the center of the Material Plane, which is as massive as a star. It's a comptetly mindless entity, which is not aware of the cosmos, nor its own existence. Powerful lesser gods surround it, listening to its gibberish, playing music to it and singing mad songs. Those who worship Azathoth consider these entities to be manifestations of the Daemon Sultan's will, and Nyarlathotep is the greatest of them. Those whom worship Azathoth seek to use its destructive power for their own agenda, but it can backfire disastrously, as Azathoth's tiniest thrashing destroys worlds.
Race of the Mythos
On Golarion there are races that worship the Elder Mythos and one race of aberrations are the offspring of an Outer God. Across the cosmos are several races that serve the Elder Mythos or are known to those that study the Elder Mythos.
Spacefaring Mythos Races
Mi-go = Countless worlds across the galaxy have been colonized by them and the number of Mi-go is incalculable. Masters of both magic and technology, they've merged the two. Impossible devices that alter reality have been created by the Mi-go. Wherever there is a world that has sentient life that interests the Mi-go, they infiltrate it's societies and manipulate the world's inhabitants to better serve the Mi-go's agenda. The goal of the Mi-go is to eventually become the masters of the universe.
Elder Things = It's the most powerful spacefaring race, having colonized countless worlds throughout the cosmos. One of the Elder Things' interests is creating life, therefore they prefer planets that lack sentient lifeforms. Whenever they discover such a planet, they are willing to go to war against other spacefaring and extra-dimensional forces for the right to possess that world. Elder Things also have a passion for architecture and creating art. They are able to fly across space without starships, but it takes them months to travel across a solar system or 1000s of years to reach the next star system. Therefore during the trip they go into hibernation. If an Elder Thing remains too long on a world (e.g. eons), it loses the ability to survive within the vacuum of space. On many worlds the Elder Things have created the Shoggoth, and on many of those planets the Shoggoth have rebelled and destroyed their colonies. Biologically the Elder Things stand 6 ft tall, have a 7 ft wingspan, and possess psionic abilities.
Star Spawn = It's a race that has only one agenda, which is to end the universe. To achieve that, the Star Spawn indirectly influence populations and individuals, or directly control apocalyptic cults. Those who serve the Star Spawn earn only one reward, which is to be the last to perish. Star Spawn don't hate other sentient life forms, to them other life forms are just an obstacle to the great plan and therefore they must die. It's nothing personal, nothing must stand in the way of the coming End Times.
Mythos Races of Golarion
Neothelids = A race of powerful and foul worm-like creatures, that are cunning and intelligent. They are massive worms with four barbed tongues, drenched in slime. With incredibly tough hide and whose maws easily swallow smaller creatures. Millions were spawned on Golarion by Shub-Niggurath during the Age of Creation. Who then borrowed deep into the Darklands, establishing a great subterranean empire where they hunted and consumed each other for sport for 10s of thousands of years. When other races arrived in the Darklands, they were overwhelmed by these races which bred at faster rates. Which forced them to retreat, stopped hunting each other, and reproduce. Centuries later they began to expand their territory, and are currently waging proxy war with the intellect devourers. Neothelids possess various psionic abilities (telekinesis, telepathy, and teleportation) and they project a code of acid from their maw. Only powerful spellcasters can harm them with their magic and their hide resist blows from all weapons except those made of cold iron. Biologically they are immortal but their Neothelid eggs centuries to hatch though they can spawn hundreds of seugathi each month, creating armies within decades. They usually live alone and when they rarely do gather, it's in small groups. Seugathi offspring are used as servants, their strongest as bodyguards, and the seugathi speak on behalf of the Neothelids.
Deep Ones = It's a race that is as ease living in an ocean trench as they are residing in a secluded shoreline's shallows, though they favor areas which combine the two. Their largest cities are located close to the shore, within huge submerged canyons. Which permits them to pursue their favorite goals, corrupting the surface-dwelling humanoids and blending with their kind. They mate with surface dwellers to spread their blasphemous faith on dry land. Hybrid children of humans and deep ones have formed their own race, which normally lives together in communities, leading and dominating small towns. Deep Ones normally worship the Great Old Ones and/or Outer Gods, with Cthulhu being foremost among those they worship. Their settlements may contain an Elder Deep One, and the ones that do worship that powerful Deep One who in turn serves an eldritch god. Clerics among them will incite among the Deep Ones religious fervor, promoting their chosen god as the greatest among the Outer Gods.
Non-Mythos Lovecraftian Factions
The Elder Mythos are not the only Lovecraftian or Cosmic Horror groups in the Pathfinder multiverse. One of which is in actual opposition to the gods of the Elder Mythos, and like the dark entities of the Dark Tapestry, they too pose an existential threat to the mortal races.
Are a race that came from another world, a horrific race of giant tentacled fish-like aquatic beings. Ancient and primordial, who are masters of mind magic and glyph (rune) magic. They are known for creating slave races and enslaving others, and establishing dark tyrannical empires. With their magic they opened portals to other worlds and spread across the stars. One of the worlds they established a colony is on Golarion. They are athiests, claiming that they predate the gods and see them as upstarts.
An eldritch race of monstrous beings, whose sub-species come in a variety of forms with tentacles and fangs. Before there were demons, the Qlippoth ruled the Abyss. When they became aware of the Great Beyond (the material universe) they surged forth, doing battle with the Proteans (serpentine chaotic beings) which gave time for the establishment of order in the Great Beyond and the creation of other dimensions. When portals would open to the Abyss, they would swarm out and prey on all.
They would eventually lose their control of the Abyss to the demons, who though weaker vastly outnumbered them. The Qlippoth were almost exterminated and were driven into deeper planes. Qlippoths are currently a pale shadow of what they once were, the Qlippoth Lords haven been reduced to power to the level of a new demon lord (though there may still be a few in the depths of the Abyss with the powers of demigods). The Qlippoth have one goal, to regain control of the Abyss, exterminate the demons, and wipe out all mortals who can sin as they are the foodstuff of demons.
Dominion of the Black
It's a galaxy-spanning and perhaps intergalactic conglomeration of races who serve a group of vast and powerful alien intelligences. These alien intelligences are similar to the Great Old Ones but do not use magic and they are beings of the material plane. Entities which are at least as powerful as the Great Old Ones and perhaps more, who have vast psychic powers. One of which is Tychilarius, a massive abomination created by the fusing of 1000s of smaller abominations in the center of a black hole. The Dominion of Black is not state but a network of worlds controlled by them, which are used as facilities to produce weapons and servitor races. Using bio-mechanical technology, cybernetics, and genetic engineering.
Worlds are conquered to provide their biotech fleshfarms with more input. Their forces include monstrous servitor races and are equipped with interstellar starships with faster-than-light capability and directed energy weapons. It's members view humanoids as inferior, good only as raw material, to experiment on, and to manipulate their genes. Scholars state that the Dominion of the Black is in actual opposition to the Elder Mythos. No one knows their agenda, but it's rumored that their ultimate goal is the extermination of all life in the cosmos and/or its replacement with something else from another reality.