When Jorge Rivera-Herrans released part of Epic: the Musical last Christmas, he managed to push Taylor Swift off the top of the US iTunes album charts. So there is a lot at stake when the final instalment of his musical retelling of the Odyssey is released on Christmas Day.

Rivera-Herrans’s project has already been an extraordinary success, with more monthly listeners on Spotify (1.6m) than veterans such as Morrissey, Liam Gallagher, or the Sex Pistols, and 119m plays on the platform in the past 28 days alone.

“I wanted to have sword fights and the ocean, and I wanted to have gods and monsters, and spells and love and lust and revenge,” he told the Observer. “I want people to have this feeling of wonder, that people can watch it and feel like a child again.”

Epic is a musical, but not a theatre production. At least, not yet. It is a 40-track concept album with Rivera-Herrans singing the part of Odysseus on his 10-year voyage home to Ithaca after the siege of Troy, with each step released on TikTok.

Epic feeds two obsessions of teenagers at the vanguard of generation Alpha: Greek mythology and fan participation.

Rivera-Herrans began writing and recording in his bedroom studio, later building a soundproofed vocal booth with his father. While most artists are determined not to risk spoiling their magic by revealing creative secrets, Rivera-Herrans is the opposite. He has shared everything from song motifs to orchestration choices and the audition process.

“I was terrified at first,” he said. “The first time I put a video on TikTok I was so nervous I did not sleep that night. But it’s one of the best things I’ve ever done, because what’s so cool about getting to show the process online is that we’re all on an odyssey together. You get to see in real time what works and what doesn’t.”

The first songs were solo affairs, but Rivera-Herrans then held auditions on TikTok, with candidates posting their own videos singing along to his music. “I thought we were going to get maybe 30 auditions, but we had 1,000 video submissions by the end of the month,” he said.

Fans have also created their own animations to bring Epic’s songs to life and Rivera-Herrans relishes their interactions. “If I tried dropping a hint of something happening in an earlier song – I do a lot of clues through musical motifs – will people pick up on it? When they do, it’s so rewarding.”

Perhaps the most obscure leitmotif spotted by fans is a trumpet melody that signals Poseidon is responsible for the storm that keeps Odysseus and his crew at sea for years, without the god appearing in the scene. It’s only later on that the theme returns, sung by the god.

“It’s so awesome that people were able to [work it out],” Rivera-Herrans said. “We’re figuring out cool ways of doing storytelling as we do this, and it’s so exciting.”

Fan participation in musical theatre has been growing since composers began sharing work on YouTube in about 2015, according to Clare Chandler, a senior lecturer in musical theatre at University of Lincoln’s school of creative arts.

Be More Chill, a show about an awkward high school student trying to become cool, originally produced at a theatre in New Jersey, became a minor hit when the cast album was picked up by Spotify’s algorithm. After gaining momentum online, it sold out off-Broadway, where people came “from across the world to see” it, Chandler said, and then it moved to Broadway. “[It] goes from something that was disregarded, through this virtual Broadway environment to be something that becomes staged on Broadway because of its popularity.”

The pandemic fuelled the rise of two other TikTok musicals. First came Ratatouille, which emerged from the online meme culture that had grown around the Pixar movie. A variety of TikTok users composed songs and the Ratatousical ended up on Broadway for a one-off charity show.

Then Abigail Barlow and Emily Bear created the Unofficial Bridgerton Musical after Barlow posted a video of her singing a fragment of a verse. They won a Grammy for best musical theatre album, but attempts to actually stage the show prompted a lawsuit from Netflix.

The question Epic fans have been asking all along is whether they will get to see it on stage.

After the final saga drops on Christmas Day, when Odysseus finally arrives in Ithaca, they may get the answer. Rivera-Herrans and his team are in talks with what they describe as a “very high- level company” to make an animated movie, and with another to create a live action stage show. Three video games are planned, with two already on the way. And the team is conscious that fans will want to be involved in the process of bringing these things to life.

“What is the next version of Epic that we put out into the world? I’m so open for all of the options, because in each different version we can convey different aspects of the story,” Rivera-Herrans said.