It doesn’t happen often that fanfic rises to this level of success, but SenLinYu’s upcoming novel has landed one of the biggest movie rights deals ever for a debut book.

Legendary Pictures has picked up Alchemised in a preemptive seven-figure deal, meant to head off competing bids before they could even get started. Industry chatter puts the outright purchase north of $3 million, which puts it among the largest sums ever paid for film rights to a book. Legendary declined to comment on the details, but the move comes as the studio rides high on its new partnership with Paramount and the nearly billion-dollar success of Minecraft: The Movie.

“I’m honored by Legendary’s incredible enthusiasm for the project and can’t wait to see the world of Paladia come to life,” SenLinYu told The Hollywood Reporter.

SenLinYu (who uses they/them pronouns) is hardly an unknown. They rose to prominence in the Harry Potter fan fiction community, especially with Manacled, a dark, dystopian take on Dramione (Hermione/Draco) that drew millions of readers on AO3 and went viral on Goodreads. Their work has been downloaded more than 20 million times worldwide, translated into 23 languages, and even extended into award-winning Star Wars fan fiction.

As staggering as all this is, the decision by Penguin Random House to publish Alchemised in the first place had strong foundations. Sen’s immense following was the social proof Penguin Random House needed to publish SenLinYu’s book in the first place, and a great many decisions by traditional publishing as to whose debut novels to publish is grounded in this kind of established fandom. SenLinYu came prepackaged with incredible bonafides.

Alchemised itself began life as Harry Potter fanfic before Sen reshaped it into an original dark romantasy. It follows Helena Marino, an alchemist and healer with a fractured memory, sent to the estate of a powerful necromancer who seeks the secrets she’s forgotten. The book promises a clash of memory, identity, and survival in a world of necromancers and corrupt guild families.

Del Rey will publish the novel September 23, with an initial print run of 750,000 copies and translations lined up in 21 languages. It’s already been spotlighted by outlets including The New York Times, Today, and The Washington Post.

Hollywood has seen this trajectory before. Fifty Shades of Grey began as Twilight fan fiction before becoming a publishing and box office juggernaut. Alchemised may be on a similar path.