As detailed and well thought out as J.K. Rowling’s Harry Potter series was, the magical wanderings of the Boy Wizard have still left plenty of room for interpretation, despite some serious effort on the author’s part to complete the story. Her digital addendum to the series, Pottermore, has helped to clear up certain lingering questions about what went down before, during, and after Harry’s tenure at Hogwarts. Meanwhile, she’s also gifted fans with the book-ending stories contained in the Fantastic Beasts and Where to Find Them films and her play, Harry Potter and the Cursed Child!
One process that never fully gets explained in the Harry Potter series is how horcruxes are made. Lord Voldemort was able to make himself a bounty of these little life preservers at some point in his younger years, but he seemed to be the only one who ever had any backup souls to spare and seemed to be unable or unwilling to create any more once our heroes started destroying them, so it must’ve been difficult.
J.K. Rowling herself has been vague about how they’re made, though she did once say that attempts would probably be deadly for most wizards because it’s as complicated as splitting an atom. Based on certain peoples’ reactions, we have to infer it was also a pretty gross process, which has one Redditor to surmise that the fous Cursed Hope Diamond may be involved somehow. The idea is that both hope diamond and the creation of horcruxes involve stealing a life force for the absorption of another, so perhaps Voldemort simply stole people’s essences to break off a piece of himself for later in their place. The theory seems to have gained some serious traction in Fantastic Beasts: The Crimes of Grindelwald, as it was revealed that prior to becoming Voldemort’s pet and tag-along horcrux, Nagini was actually a shape-shifting maledictus who had a female human form before we met her in the original series. Voldemort may have killed off the piece of Nagini that was human and swapped in the Famous Cursed Hope Diamond.