Dumbledore had a younger sister Ariana, who was assaulted by three Muggle boys when they saw her doing magic.
Because of this, Ariana refused to use her magic again, and becomes magically unstable.
Years later, their mother Kendra died in an outburst of magic from Ariana which was totally accidental.
Now, after fans saw the Fantastic Beasts movie, they believe they uncovered why Ariana was so dangerous – she was an Obscurial.
She's a what?
A specific Obscurus played a MAJOR part in Fantastic Beasts...
An abused teen wizard, Constance, is wayyyy too scared to use his powers, so he also becomes magically unstable, and internalised his powers.
He develops an Obscurus, which is a dark force that bursts out of him, eventually killing his adopted mother and causing a huge mess in NYC.
Fans have noticed that his story and Ariana's are too similar to be coincidence.
Aberforth (Albus' brother) explains in the final Harry Potter book that Ariana's magic "turned inward and drove her mad, it exploded out of her when she couldn't control it,” and their mother Kendra was killed because of it. Yikes.
In Fantastic Beasts, Dark Wizard Gellert Grindelwald (Dumbledore's best friend, turned boyfriend, turned enemy) tries to use Constance as a weapon.
He see's what happened to Ariana in the summer that he spent in Godric's Hollow with Dumbledore... so he may have used that knowledge to manipulate Constance.
Fans have asked J.K. Rowling about this theory, but she has yet to make any comments on it.
The second Fantastic Beasts movie will come to theatres November 16, 2018!