For the foundational myth of the Warhammer 40,000 universe, the Horus Heresy had an extremely prosaic origin. The reason it exists, this massive fallen-paradise fairytale taking place 10,000 years in the setting's past, is that Games Workshop couldn't afford to put miniatures representing two different factions in a box back in 1988.
The Realm of Chaos supplements, the first published the same year as Adeptus Titanicus, detailed the Horus Heresy in broad strokes. A two-page short story accompanied by a black-and-white art piece by Adrian Smith from the second volume depicted the climactic duel between Horus and the Emperor, and other tellings followed in art books and magazine articles. But it was mostly broad-strokes stuff. We didn't get more of the specifics until the 21st century.
To be fair those 54 Horus Heresy books aren't all novels, with a substantial number being anthologies of extremely skippable short stories tangential to the main story, and we only get to 10 Siege of Terra books because the finale, The End and the Death, was so long it had to be released as three separate volumes. Still, whichever way you count it, and even if you ignore the comic book and audio dramas, the thing as a whole is an extraordinarily long piece of work.
And though this version isn't exactly the same as it was in previous tellings, it's remarkable how much of the old lore remains. A contradiction in earlier versions has been who gets the job of bursting into the chamber to get in the way of the duel right before it finishes, which is handled here by letting multiple people be That Guy, one after the other, and explaining them as part of the Emperor's scheme.