At the time the trailer hit the internet, Entertainment Weekly revealed that Kim kept her role as Nagini a secret for more than a year - which was no easy feat considering the amount of fan theories she stumbled upon since being cast.
In this prequel to the Harry Potter series, we're heading back to the Wizarding World following the escape of Gellert Grindelwald (Johnny Depp). Nagini, for those of you not up on your Potter lore, later becomes bonded to Lord Voldemort, becoming one of his Horcruxes (and thus containing a piece of his soul). Nagini does have some measure of control over her transformation in the trailer; she may not know the scope of her condition or will experience physical consequences of transforming regularly.
Also, the EW article linked above says Nagini "has powers like wizards have".
The above promo indicates that their quest will involve land, sea and air skirmishes, as well as pit them against Grindelwald's evil minions - including, most exciting of all, Nagini (Claudia Kim), a shape-shifting "Maledictus" circus performer who's recruited by Credence Barebone (Ezra Miller) and who, it turns out, is the future pet snake of Lord Voldemort. In this, she's a wonderful and vulnerable woman who wants to live.
She added: "She wants to stay a human being and I think that's a wonderful contrast to the character". She is bound to [permanently] transform at some point to a beast so she feels this pressure that the clock is ticking."She has powers that are yet to be explored".
We also get a good look at Newt's brother Theseus Scamander, as played by Callum Turner. But there's something more troubling about this twist. Yeah, this may be an even worse retcon than making Dumbledore gay and refusing to follow up on it.
Fantastic Beasts: The Crimes of Grindelwald is out in the United Kingdom on 16 November 16, 2018.