Fantastic Beasts: The Secrets of Dumbledore can finally reveal Harry Potter's darkest mystery surrounding Albus Dumbledore's (Jude Law) sister, Ariana. The third Fantastic Beasts movie will see Aberforth Dumbledore (Richard Coyle) reunite with his older brother Albus, who is rallying everyone around him (including Eddie Redmayne's Newt Scamander) in his war against his friend-turned-foe Gellert Grindelwald (Mads Mikkelsen). It has been a long time since the brothers spoke to each other, and this reunion is bound to explain the tragic story of Ariana Dumbledore.
When she was six years old, Ariana was violently attacked by three Muggle boys, an event which deeply scarred Ariana and led her to suppress her magic, much like Credence (Ezra Miller) has been forced to do in his youth. In Harry Potter And The Deathly Hallows, Aberforth explains that Ariana's magic "turned inward and drove her mad, it exploded out of her when she couldn't control it" - this once again reminds of Credence, even leading to the theory that Credence is possessed by Ariana's Obscurus. The Dumbledores' father, Percival, eventually went to Azkaban for attacking the Muggle boys, and their mother, Muggle-born Kendra, was accidentally killed by Ariana in one of her outbursts. Albus was then tasked with protecting Ariana, but that didn't go well either: during the famous three-way duel between the Dumbledore brothers and Grindelwald, Ariana intervened to stop them and lost her life. This drove Aberforth and Albus apart, as Aberforth blamed Albus for the tragedy.
Fantastic Beasts: The Secrets of Dumbledore will see the brothers reunite perhaps for the first time since Ariana's funeral (where Aberforth broke Albus' nose), and considering the title of the movie, Ariana's troubled past is bound to come out. In the trailer, Aberforth asks Albus - with a sarcastic undertone - if he's "off to save the world." Although harboring some resentment, Aberforth wants Grindelwald (who has been recast in Fantastic Beasts 3) dead just as much as his older brother. While it's still unclear whose wand cast the spell that killed Ariana, when Harry meets Albus in King’s Cross after the Battle of Hogwarts, the latter hints that it was Grindelwald's. It thus makes sense that Ariana's death is fuelling both Albus and Aberforth's efforts against Grindelwald. The Secrets of Dumbledore will have to explore Ariana's tragic story if it is to explain the feelings of blame and grief that both her brothers carry throughout their lives.
Ariana's death is indeed a very big part of Albus Dumbledore's life, as well as his death. J.K. Rowling confirmed that when Dumbledore told Harry he saw socks in the Mirror of Erised, he actually saw his family reunited. And when Dumbledore goes hunting for Voldemort's seven Horcruxes, he finds Marvolo Gaunt's ring, which he realizes contains the Resurrection Stone, and even though he is fully aware the ring is cursed, he puts it on. He explains his motivation also at the end of The Deathly Hallows: "I put it on, and for a second I imagined that I was about to see Ariana, and my mother, and my father, and to tell them how very, very sorry I was." The guilt Albus carried with him for Ariana's death ultimately led to his own death - this is Dumbledore's dark secret, and a Harry Potter mystery that the new film ought to develop upon.
Although Fantastic Beasts: The Secrets of Dumbledore has been criticized for being a cheap trick meant to attract a wider audience with a fan favorite (losing focus on Newt in order to return to the popular Dumbledore), the Dumbledore family secrets are crucial to explaining the war against Grindelwald. It's important to explain Aberforth's motivation in joining his brother in battle. And with the major plot twist (and canon addition) that Credence is actually Aurelius Dumbledore, a fourth sibling, there must be one film in the five-movie franchise that clears Harry Potter's darkest mystery around Ariana and the Dumbledore family.
