Each trailer for Fantastic Beasts: The Crimes of Grindelwald has unfolded a new piece of lore for the Wizarding World. The first trailer, when it arrived back in March revealed hints of the French Wizarding World. It also suggested assumptions about Hogwarts, like it's apparition defenses, were not always there to be taken for granted. The second trailer, at San Diego Comic-Con, revealed Dumbledore once taught Defense Against The Dark Arts. The final trailer's big reveal was Nagini was once human, and she's not like anything Potter fans have seen before. So what is the difference between a Maledictus and an Animagus? After the big reveal, J.K. Rowling took to Twitter to explain.
Animagi were introduced in the first Harry Potter novel, in the opening prologue, as McGonagall, as a cat, is seen observing the Dursley household. From the beginning, the idea humans could change form into animals has been a running theme in the Rowling Wizarding World.
But in this type of spell, the wizard or witch transforming from an animal and back has total power over the process. Even if they turn themselves into inanimate objects, like teapots, they are always in control of the transformation, and the return to human form.
In the middle Harry Potter novels, as Voldemort's rise continues and Nagini becomes his beloved sidekick/pet, characters observe the snake seems strange, almost, overly intelligent. By the end of the series, this has been explained away by declaring the snake a Horcrux, and the human-like cunning the creature had was really a reflection of Voldemort's soul in her eyes.
The newest trailer from Fantastic Beasts: The Crimes of Grindelwald, takes this belief and sets it sideways. It turns out Nagini is, in fact, a Maledictus, a different type of transformational magic, one where the human who is changing shape does not have control.
According to the Harry Potter wiki, this magic first turned up in Harry Potter & The Cursed Child. A "blood malediction" is a curse that resurfaces in a victim's descendants, sometimes lying dormant for generations, sometimes passing directly from mother to daughter. In Cursed Child, Scorpius' mother, Astoria Malfoy had an active malediction, though what it is precisely is never defined.
These curses are carried genetically by the mother and passed only to daughters. Anyone who bears an active version of the curse is known as a "Maledictus." The wiki suggests the curse only refers to those who slowly change into animals. Also, just because a mother turned into a snake does not automatically mean the daughter (or the great-granddaughter) would take on the same shape.
There have been questions if Nagini is a witch or not, or if she's just someone who bears this curse. But the Potter wiki states it is unknown at this time if a Muggle could be a Maledictus, suggesting these blood curses only have been used on those of magical breeding.
As for how long Nagini has between her human form and transforming into a snake for the final time, the trailer has not made it clear. Either way, fans already know her ultimate fate rests at Hogwarts, to die by the blade wielded by Neville Longbottom.
Fantastic Beasts: The Crimes of Grindelwald arrives in theaters on Nov. 16, 2018.