When Harry and Hermione are on the first floor of Bathilda's house, Nagini (inside the corpse of Bathilda) tells Harry to "Come!" from the next room in Parseltongue. In reaction, Hermione jumps and clutches Harry's arm, and the two of them obey the command.
Because some snakes can sense heat and movement in a way humans cannot, Nagini was able to detect Harry and Hermione even when they were under the Cloak of Invisibility. Harry fought with Nagini, and the serpent bit him before coiling herself around Harry to hold him in place for Voldemort.
Edit: after writing this I realized Harry DOES talk to Nagini when she's inside of Bathilda, he just doesn't realize it.
After Harry and Hermione's encounter with Nagini in Godric's Hollow, Harry says: "She didn't want to talk in front of you, because it was Parseltongue, all Parseltongue, and I didn't realise, but of course, I could understand her." But Hermione did hear her talk. "Come!" called Bathilda from the next room.
You'll learn a lot more about him in the second half of the movie. What's up with that snake lady? Bathilda Bagshot has actually been dead for weeks, which explains the flies and blood handprints Hermione finds.
Bathilda was murdered during the height of the Second Wizarding War through the Dark Arts and her body was animated by Lord Voldemort transforming her in Nagini. For this reason, Bathilda's body could technically be considered as an Inferi.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanetly. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.
Nagini as Bathilda Bagshot speaking to Harry Potter in Parseltongue Sometime after her interview with Rita Skeeter, Bathilda was murdered through the Dark Arts and her body was animated by Lord Voldemort. Voldemort did this using his snake, Nagini.
The snake was planted in the animated corpse of Bathilda Bagshot to wait to see if Harry would come to see the graves of his parents in Godric's Hollow, and then to hold him there until Voldemort arrived to murder him.
Nagini is loyal to Voldemort because he can speak with snakes. As a Maledictus, there probably isn't much time after the events in The Crimes of Grindelwald before Nagini falls to her blood curse, becoming a snake forever.
Snake Harry set free and Horcrux victim
"There was a rumour that Nagini was the snake Harry released from the zoo in Harry Potter and the Philosopher's Stone. This was proven false, as Nagini is some type of viper while the snake in the zoo was a boa constrictor.
Nagini is one of many snakes featured in the series
After all, the great Basilisk in the Chamber of Secrets was a huge snake, bred with Salazar Slytherin's intent to murder Muggle-borns. The Dark Mark, Lord Voldemort's 'sign', was a skull with a snake tongue.
Harry did not know that he was one, and no one knew that Nagini was one. Dumbledore only revealed the truth about Harry and Nagini after Harry's "death", when they talked in the spirit realm. So at the time of the conversation, they only knew of one more Horcrux, the fifth one, the diadem.
But Rowling put a clever explanation into her story for why Aurelius does not exist in the Potter books at all. It's in his name. Marcus Aurelius was the last of Rome's "Five Good Emperors" and is overlooked a lot in the history books in favor of those who came before and after.
The ability to speak Parseltongue is usually inborn; a wizard is born being able to speak Parseltongue, and those who are not born with the ability cannot normally learn the language (although, according to the author, Dumbledore may have learned to understand it, and in Harry Potter and the Deathly Hallows, Ron learnt ...
Contains SPOILERS for Fantastic Beasts: The Secrets of Dumbledore. Fantastic Beasts: The Secrets of Dumbledore finally revealed Credence (Ezra Miller) as Aberforth Dumbledore's (Richard Coyle) long-lost son, but his mother can very well be a Gaunt, making Credence related to Voldemort (Ralph Fiennes).
"I wish you were working with me now. Towards a world where we wizards are free to live openly, and to love freely,"
Case study 2: Severus Snape
A doe. And in his final battle with Lord Voldemort, Harry explained the significance of this to his adversary, and to us: 'Snape's Patronus was a doe,' said Harry, 'the same as my mother's, because he loved her for nearly all of his life, from the time when they were children'.
In “The Crimes of Grindelwald,” set in 1927, Nagini is part of a traveling magical freak show, where she entertains the crowd by turning into a snake. As it happens, she is a human witch who suffers from a blood curse called Maledictus.
If you can see/feel what your Patronus sees/feels when you send it off on a mission, Snape would know the moment the Patronus found Harry and would be able to hide the sword in the lake in time for Harry to find it when he arrived, minutes later.
Phineas went to his Hogwarts portrait and relayed that information back to Snape, who then traveled to the Forest of Dean and planted the sword.
It was mentioned that Harry and Ron thought there was someone there with them. It's implied that the same person who cast the doe patronus put the sword into the lake.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
Norris being a Maledictus is her relationship with Hogwarts staff member and caretaker Argus Filch. The two are introduced together in both the Harry Potter movies and books.
"In her name — because the Naga are, in mythology, a race of snake beings. So the name was an allusion to the fact that she may herself once have been human." Nagini is revealed as one of Voldemort's horcruxes in "Harry Potter and the Deathly Hallows." Warner Bros.