When it was revealed Nagini would make an appearance in Fantastic Beasts: The Crimes of Grindelwald, we were intrigued, but after seeing the movie — and knowing that (spoiler alert!) she chooses not to follow Grindelwald like friend Credence Barebone, aka the lost Dumbledore — we have questions.
After Newt Scamander and the Aurors' battle against Grindelwald, Nagini goes with them, the good guys, to Hogwarts. Somehow, 67 odd years later, Nagini ends up hanging out with the bad side, becoming one of Voldemort's most powerful weapons from Harry Potter and the Goblet of Fire to the end of the series. How the tides turn! If you're wondering why the Maledictus made a total 180-degree turn, you're not alone. Let's investigate why she may have slithered her way into the dark side.
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. The Crimes of Grindelwald ends with Nagini at Hogwarts, so it wouldn't be far-fetched to assume that she stays there and perhaps crosses paths with Tom Riddle when he's a student at the school from 1938 to 1945. (Side note: Voldemort's last school year takes place when Dumbledore defeats Grindelwald.) Voldemort's ability to speak Parseltongue may then be the reason she befriends him, since being trapped in snake form would isolate her from communicating with other people. Perhaps she even helps him open the Chamber of Secrets and summon the basilisk.
Voldemort possesses Nagini because she's immortal.
YouTuber Seamus Gorman points out that Nagini's Maledictus curse could make her immortal, and we all know that Voldemort's thirst for power would have loved that. The Dark Lord mentions to his Death Eaters in Goblet of Fire that he only had one power left when hiding in the forests of Albania, that of possessing others, especially snakes, which he disliked because they didn't have magical powers. If he had possessed a Maledictus, a person of magical descent cursed to permanently become a beast, he would have more power. This would explain how Nagini came to be the perfect horcrux.
Nagini is Voldemort's mother.
This theory is out there, right? The Harry Potter series tells us that Voldemort is Tom Riddle Sr. and Merope Gaunt's son, but let's humour this mind-boggling Reddit theory from user soupycutie. The Crimes of Grindelwald takes place in 1927, and Voldemort is born late in 1926. What if Nagini had given birth to Voldemort shortly before popping up in Paris to perform at the circus? It's curious that Mrs. Cole, the woman who kept Tom Marvolo Riddle at her orphanage, wondered whether his mother had come from a circus. Additionally, Wormtail "milks" Nagini so that Voldemort could have her venom to become his creepy, snakey baby form in The Goblet of Fire. In any case, Merope might have had a connection with Nagini — maybe they bonded over the former's Parseltongue ability.
Author J.K. Rowling drops so many Easter eggs in The Crimes of Grindelwald script that I'm certain she'll find a way to tie Nagini and Voldemort together in the Fantastic Beasts franchise. Hopefully, Claudia Kim, the actress behind Nagini's human form, will have a more nuanced role to play in movies to come.