It took 12 years to crack the code. Since the release of Assassin’s Creed II in 2009, fans have puzzled over the mysterious language of the Isu, powerful beings who, in the series’ convoluted lore, created the human race long ago. Their alien glyphs have appeared throughout the 11 games in the main series, but remained indecipherable until the latest instalment, Assassin’s Creed Valhalla, provided players with a digital Rosetta stone — documents with English and Isu side by side. Using these, the fan community was able to extrapolate the rules of the Isu alphabet and grammar, surprising the game designer who created the language, who tweeted: “I didn’t even think it would be possible to break it down. Mad skills!”

Isu is a constructed language, or “conlang”, a term denoting a language whose vocabulary, grammar and phonology have been consciously devised rather than developing organically. One of the earliest conlangs, the lingua ignota, was created by the abbess Hildegard of Bingen in the 12th century as a secret language for her nuns. Since then, writers ranging from Dante to Paul Auster have written about constructing a perfect, pre-Babel language, a universal tongue.

Conlangs are created for several reasons. Esperanto was meant to ease human communication, while Robot Interaction Language is algorithmically calibrated to minimise confusions when humans speak to machines. Yet many of the best-known conlangs are artistic in nature, devised to add a layer of realism to fictional worlds. “Conlangers” are a passionate online community who decode and promote these languages. They have their own flag and Cambridge conference, and communicate largely in intellectual posts found in unusually polite corners of the internet.

The father of conlangs is indisputably J.R.R. Tolkien, a prominent linguist who drew from Finnish, Welsh and Ancient Greek to craft 15 languages for The Lord of the Rings, including Elvish, Dwarvish and the Black Speech of Sauron. From here, all other conlangs flowed: the Na’vi language in Avatar, Parseltongue in Harry Potter, Dothraki and High Valyrian in Game of Thrones, the latter of which can today be studied on language app Duolingo.

These range in complexity. Some written conlangs are simple “substitution ciphers”, swapping Latin letters for an invented alphabet without devising an alternative grammar. Other conlangs have a limited grammar and vocabulary intended to gesture at a full language, like a stage set. The most ambitious conlangers create entirely new linguistic systems with their own history and character — in Game of Thrones, for example, it is fitting that the warrior Dothraki tribe have no word for “thank you”.

The most famous conlang is probably Star Trek’s Klingon, which has been codified in a dictionary and used to translate Hamlet and the epic of Gilgamesh. D’Armond Speers, linguist and member of the Klingon Language Institute, tried to raise his son as a native speaker by only speaking to him in Klingon, but abandoned the project after a few years when the child gravitated towards English despite, Speers noted, his superlative pronunciation of Klingon’s guttural phonemes.

Introducing conlangs brings two main benefits to these series. For average viewers, the languages make fantasy worlds more immersive and authentic. For hardcore fans, they are codes to crack and share with other devotees. It’s only natural that as video game worlds became more sophisticated, developers would want their own constructed languages, too.

The Elder Scrolls: Skyrim introduces Dovahzul, the language of dragons and magic spells, with a 34-character alphabet made of scrapes and dots, the only shapes a dragon might reasonably be able to carve into stone. Far Cry Primal, set in the stone age, includes two languages that approximate the proto-Indo-European spoken by our ancestors 12,000 years ago. These are used to voice the entire game by actors coached to speak and emote in ancient tongues.

The conlang created for the 2005 game Jade Empire was particularly sophisticated. Tho Fan was the aristocratic language of the game’s fantastical eastern setting, crafted by a PhD student over four months for a budget of about $2,000. The student tested his 2,500-word vocabulary by translating the first chapter of St John’s Gospel before submitting it to the developers. It was only last autumn, 15 years after the game’s release, that the conlang community finally cracked the Tho Fan code.

For many gamers, the best-known fictional language is Simlish, the garbled noises made by characters in The Sims when they chat, flirt, or urinate on the floor because you forgot to build a toilet. Musicians from Katy Perry to Lily Allen to Depeche Mode have re-recorded their hits in gobbledegook for the game’s radio stations over the years. Yet Simlish is not technically a conlang. It is actually gibberish, invented spontaneously by voice actors using an improvisational comedy technique of telling a story using only nonsense sounds. Some of their key phrases have been enshrined in Simlish vocabulary across the series — “sul sul” means hello, “nooboo” is child — but fans have never succeeded in translating Simlish because it lacks both consistency and internal logic as a language.

Each conlang demands labour, time and money to create. Given that only a fraction of fans bother to decode them, is their inclusion really worth the effort? Based on the evidence, the answer is yes. Media series featuring conlangs include many of the most beloved and lucrative films and games of all time.

In today’s sci-fi and fantasy landscape, there are two keys to success — world-building and engaging the hardcore fan base. Conlangs neatly achieve both. In the only book he published in his lifetime, Wittgenstein wrote that “the limits of my language mean the limits of my world.” By setting the human imagination loose on language, these creators are opening the gates to worlds without limits.