Mythopoeia

Mythopoeia (from the Greek words that mean "myth-making") is a narrative genre in modern literature and film where a fictional mythology is created by the author or screenwriter. The word mythopoeia and description was coined and developed by JRR Tolkien in the 1930s. The authors in this genre integrate traditional mythological themes and archetypes into fiction. Mythopoeia is also the act of making (creating) such mythologies. Notable mythopoeic authors aside from Tolkien include CS Lewis, Robert W Chambers, HP Lovecraft, George MacDonald, and Lord Dunsany, among others. While many literary works carry mythic themes, only a few approach the dense self-referentiality and purpose of mythopoeia. It is invented mythology that, rather than arising out of centuries of oral tradition, is penned over a short period of time by a single author or small group of collaborators.

As opposed to fantasy worlds or fictional universe aimed at the evocation of detailed worlds with well-ordered histories, geographies, and laws of nature, mythopoeia aims at imitating and including real-world mythology, specifically created to bring mythology to modern readers, and/or to add credibility and literary depth to fictional worlds in fantasy or science fiction books and movies.

Mythopoeia can be created entirely by an individual, like the world of Middle-Earth, or can be formed as a result of an amalgam of writings, like the Cthulhu Mythos. An Expanded Universe can result in the creation of one of these, particularly for Long Runners.

Comic Books

 * The Sandman, though it borrowed elements from the "parent" DC Universe (as well as many mythologies). And Neil Gaiman seems not to have planned it all in advance.
 * Jack Kirby's New Gods.

Film

 * Star Wars - George Lucas has explained that he was heavily influenced by The Hero With a Thousand Faces by Joseph Campbell - which describes how to do this in detail
 * Avatar - the Na'vi mythos has a strong resonance with many of the film's fans - to the extent of some becoming adherents.

Literature

 * The Silmarillion contains the mythic backstory for Middle-earth.
 * Furthermore, Tolkien actually created Middle-earth's histories as a mythology for the United Kingdom, since he was really torn up about the Brits not having one.
 * Lord Dunsany's The Gods of Pegana forms a complete cycle of myths, from Creation to The End. Complete with multiple contradictory versions of The End. Dunsany's mythology predates The Lord of the Rings and has a completely different feel.
 * The Cthulhu Mythos created by HP Lovecraft and others.
 * The myths and legends present in Watership Down.
 * Wheel of Time uses many real world myths and legends in its work, from African to Norse. Its implied, though never outright stated, that the world may be the same as our own through these myths.
 * Word of God has stated it, and it is very strongly implied in the text. The books say that time is cyclical, and a handful of garbled myths and fragmentary legends have persisted from the Age before last. All of them correspond to important figures from the 20th century or earlier, allowing for linguistic drift in the names of individuals.
 * The Discworld series has become this over the years.
 * The Queens Thief series by Megan Whalen Turner.
 * The Riftwar Cycle by Raymond E. Feist has an extensive mythology, which has gotten tangled up in its history at several points.
 * The Oera Linda Book.
 * The Dark Tower with the titular Cosmic Keystone being.
 * The term is often used to discribe the narrative poems of William Blake and his complex system of gods and demigods.
 * CS Lewis has The Chronicles of Narnia and The Space Trilogy.
 * George MacDonald's fairy tales and fantasy stories were cited by C. S. Lewis as arguably the Trope Maker. MacDonald's works had an acknowledged influence on later mythopoeic authors including Lewis and JRR Tolkien, making him at least the Trope Codifier.

Live Action TV

 * Babylon 5: The Minbari and the Narn cultures are the best examples of mythmaking on this show.
 * Battlestar Galactica: Both the original and the reboot feature this.
 * The Stargate Verse creates some of its mythology from whole cloth, but also integrates aspects of real-life mythology into the story. The best example is probably the altered Camelot mythos in seasons nine and ten of Stargate SG 1.
 * Star Trek: There are hints at the Myths of various races. Vulcans and Klingons are most noticed.

Tabletop RPG

 * Nobilis, a Tabletop RPG centering on the machinations of beings for the most part above our ken. The game designer also writes the web fiction Hitherby Dragons which also has a mythology of sorts.
 * Exalted, with its involved cosmological backgrounds.
 * Glorantha from Rune Quest: arguably the biggest, greatest, most complex example of this trope has somehow been left out.

Toys

 * Transformers has a considerable background mythos including a creator god (Primus) and a Satanic figure (Unicron).

Video Games

 * The PS2 Summoner games.
 * Dragon Age was made to present a fully developed world right from the start of the first game. There are references to past events and people all the time and many characters that are foreigners to Ferelden, with the natives reacting to them depending on the relations of their countries.
 * Touhou is going this way, it's build upon (mostly-Japanese) folklores on one end and the fan creating (staggering amount of) fan materials to cover the holes on the other end. Sounds familiar, hm?
 * Rayman - Starting in Rayman 2, with Polukus and the Lums, and going deeper in Rayman 3, where we are presented to the dark side of the Lums. Rayman 3 also has an ancient desert people known as the Knaaren, who worship the Leptys (also known as the Bringer of Night).
 * Rayman Origins seems to go into this even further, establishing Rayman's role as The Chosen One.
 * The Elder Scrolls - This is the primary appeal of the series story, where the free exploration is the primary gameplay appeal. There are several divergent mythologies, creation stories, and divine histories, and of course All Myths Are True to at least some degree. This is also sprinkled liberally with mythic and biblical symbolism up the yin-yang.
 * It is an actual in-universe force as well. The fabric of reality in the Elder Scrolls universe is malleable to those who possess the arcane knowledge, and one can become a god by "walking like them until they must walk like you".
 * Brutal Legend has a mythology spanning from the creation of the Heavy Metal world, through the rise and fall of Ormagöden, rise and ascention of the Titans, enslavement of humanity by the demons, the Black Tear Rebellion, to the events taking place in the game itself. Read more here.
 * The Legend of Zelda first gained a fictional mythology with A Link to The Past, which introduced the Three Golden Goddesses as the creator deities, along with the origin of both evil in general (men warring over ownership of the Triforce) and Ganon in particular (a cunning human thief who got the Triforce and was subsequently sealed in the Sacred Realm / Dark World). Ocarina of Time went into further detail, fleshing out the individual Goddesses and the nature of the Triforce while also portraying the Start of Darkness for Ganon/Ganondorf. Later games have brought the focus away from the central Triforce myth to flesh out the broader Hylian mythos and pantheon.
 * Skyward Sword takes this even further, by establishing life prior to the founding of Hyrule and the wars that sprung up even before that. Most notably, it also delves into the origins of the Master Sword and introduces both the goddess Hylia, the one who in ancient times defended the Triforce against demons and, as well as.
 * Final Fantasy XIII's actual plot focuses more on 6 chosen people and how they deal with it. The background story and the lore are a bit cluttered and mishandled but if you take time reading in-depth, you will find one of the more interesting mythopeia about how the Gods decided to create the Fal'cie, which in turn annoints a L'cie.
 * Xenogears and Spiritual Successor Xenosaga have extensive cosmologies.

Web Comics

 * Andrew Hussie's work, Homestuck, initially revolves around four kids playing a reality-altering video game that constructs a mythology around them, casting them as legendary heroes, with all the details of their world and their mythical powers tailored to their personalities and interests. What is easily dismissed as a fun quirk of the game, however, slowly develops into a more and more elaborate plot involving the fate of reality itself. Eventually the legends and prophesies becomes so pervasive that almost every event in the plot can be tied back to some part of the previously-established mythology, adding layers of bonuses for the attentive reader.
 * This is one of the draws of Ursula Vernon's long-running webcomic Digger, in which the various cultures encountered have their own myths and traditions, often combining real-world examples with facts of their world. This is most obvious with the creation myth of the hyenas, which explains both their tendency to be female-dominated and the frequency with which firstborn cubs die, both traits of real-world hyenas, but more subtle examples can be seen just in the oaths and sayings characters from different cultures use and some of the prejudices they hold.
 * Exiern has a carefully worked out mythology behind it.
 * Rumors of War: While it borrows a great deal from Classical Mythology, Rumors of War combines magic, idealism, Mood Whiplash, Loads And Loads of Arguing About Nothing in Particular into what is probably a Crossover Cosmology. Or something new entirely.

Web Original

 * Tasakeru, with the core story of the Three Gods, which is interpreted in different ways by each of the eight species.

Western Animation

 * Avatar: The Last Airbender
 * Gargoyles : Its mythology borrows from folktales and mythologies from around the world with a dose of Shakepeare.
 * My Little Pony Friendship Is Magic

Other

 * Though not created as "intentional" fiction, the Lost Continents of Mu and Lemuria were created out of whole cloth a century and a half ago, one to explain a now-discredited anthropological theory, the other to explain a now-discredited theory of continental formation, and both kept afloat by Spiritualists and Theosophists who wanted mysterious but unresearchable lands to say that their dead friends came from. It's possible that even Atlantis, at the time of its first writing, was created by Plato as a moral metaphor, not a literal location.
 * The Bionicle universe. The first few years had some influence from Maori culture, but the franchise has deliberately moved away from that and now has a complex mythology of its own. The best part is that it tends to subvert All Myths Are True by explaining that the characters tend to come up with their own explanations as to why things happen that may not be entirely accurate, making it a case of in-universe Mythopoeia.
 * Adylheim uses this extensively, not only creating an internal mythology which mimics parts of real life greek and norse pantheons, but also making references to an ambassador to faerie named Tamlin, a dragon hunter named George, and so on.
 * The Slender Man sounds like an old folktale or urban legend but was actually created whole cloth by a member of the Something Awful forums. Some writers have even tried to expand the mythos by linking it to other web-based horror entities such as Zalgo and The Rake.
 * And The Fear Mythos, which estabilished Slender Man and Rake as a part of their fear-based pantheon.
 * Websnark creator Eric Burns' Banter Latte blog had a running series devoted to creating modern-day myths. And it was awesome.
 * SCP Foundation might have gone into that territory, though we only get a small hints and glimpses of divine or demonic origin of some SCP.