With news that Fantastic Beasts and Where To Find Them 2 is casting teen versions of some of the main characters, fans of the Harry Potter universe have some tidbits to play with as we wait for the arrival of this new movie. News of the casting comes not long after J.K. Rowling confirmed that the script for the movie had been completed. What the casting reveals is that we're going to see younger versions of Newt Scamander, Leta Lestrange, Gellert Grindelwald and Albus Dumbledore. What should be clear right away, from the ages of the characters being cast, is that there are at least two different flashback (or flashback-like) sequences planned for this movie, and though both will hopefully answer questions we have about their respective characters, we're especially excited about one of them.
That we'll be learning more background information about Leta and Newt isn't particularly surprising. Given that this relationship was teased in Fantastic Beasts, and it was clear that there was more to tell about those two, more information or potential flashbacks featuring them was inevitable. Maybe the same could be said about Gellart Grindelwald and Albus Dumbledore, but given the hype built up around these two characters, dating back to the Harry Potter books, knowing that we'll be seeing them in their younger days in some form or another is a pretty big deal.
Since learning that Grindelwald was a character in this story, and that Dumbledore was set to appear in Fantastic Beasts 2, Harry Potter fans have surely speculated on how much of their backstory would be revealed -- or play out -- in these new movies, especially factoring in J.K. Rowling's involvement with the scripts. There are some basic, but massive things known about Grindelwald and Dumbledore, among which is the fact that the two formed a very brief but disastrous friendship, which began not long after the death of Dumbledore's mother, and ended with the death of Dumbledore's sister, Ariana.
Grindelwald was sixteen when he was kicked out of Durmstrang. And we know Dumbledore was fresh out of Hogwarts when he and Grindelwald met. Pottermore says the actors being cast to play young versions of these two characters should be between the ages of 16 and 18. That's a good indication that whatever we'll be seeing of young Dumbledore and Grindelwald will be during or around the time that they became friends. Beyond that, we don't know what's planned for Fantastic Beasts 2 as far as their story goes.
The story of Grindelwald and Dumbledore's friendship is told in the final Harry Potter book, but there are still some unanswered questions, including the deal with Ariana Dumbledore, and how she actually died. The working theory is that Dumbledore's sister was an Obscurus, the same condition Credence suffered from in Fantastic Beasts and Where To Find Them. Of course, there's no mention of that specifically in the Harry Potter books, but we do know Dumbledore's sister had outbursts after a traumatizing experience with a bunch of muggles who saw her using magic. That, added to what we learned about Obscurials in Fantastic Beasts has led many to speculate that Ariana might have been an Obscurus as well.
Ariana's outbursts are said to be the cause of the death of Kendra Dumbledore, Ariana's mother. That may have been what happened, but if we're dropping conspiracy theories here, I'd say that it seems like very convenient timing that Grindelwald showed up in Dumbledore's neighborhood so soon after that particular tragedy. Harry Potter canon says Grindelwald was the nephew of Bathilda Bagshot, but what if that was just a ruse to put him in the neighborhood of the up-and-coming Albus Dumbledore... or his mysterious (potentially Obscurus) sister? What if Grindelwald had some kind of involvement in what happened to Kendra? It's possible, anyway. Or things happened just as Aberforth and Albus Dumbledore said they did in Deathly Hallows. The point is, there's always the chance there's more to Grindelwald's story than even Dumbledore knew.
Even if Grindelwald wasn't involved in Kendra Dumbledore's death, we do know that he was involved in Ariana's. It happened after a disagreement between Albus Dumbledore and Aberforth Dumbledore over Dumbledore's plans to travel around the world. Such plans weren't compatible with Ariana's delicate emotional situation, and Aberforth was putting his foot down. Grindelwald got involved and put the Cruciatus Curse on Aberforth. Albus jumped into the mix, and a fight broke out. Ariana was caught in the middle, and ended up dead. Dumbledore admitted to Harry that he didn't know which of them cast the curse that killed his sister. And by his confession to Harry, it sounds like Dumbledore was terrified to learn that he might've been the one to kill Ariana. That question has yet to be answered.
Naturally, this is a story fans would want to see play out, especially if it means answering questions about Ariana's death. Whether or not Fantastic Beasts 2 will show us all of that, we don't know. In fact, they may only give us a small slice of the brief friendship Dumbledore and Grindelwald shared, and save the rest for later. After all, there's no mention of casting Ariana or a young Aberforth.