Setup
1Harry Potter and the Goblet of Fire · The house where Voldemort is hiding
A huge snake shares the room with the weakened Voldemort and is milked for venom. She is introduced as his servant and his comfort, not yet as a container for his soul.
2Harry Potter and the Half-Blood Prince · Dumbledore’s warning
Dumbledore tells Harry to remember the snake when the time comes. The warning is brief on purpose: Nagini is on the list, and Harry is not told every reason yet.
Payoff
Harry Potter and the Deathly Hallows · Godric’s Hollow, then the battle
Harry sees through Nagini’s eyes often enough to treat the link as more than Parseltongue. During the battle he understands she is the last Horcrux outside himself. Neville cuts off her head with the sword of Gryffindor.
