Voldemort is able to communicate with Nagini due to his ability to speak Parseltongue, the language of snakes. Readers are first introduced to Nagini when the snake is the first to notice Frank Bryce, an old gardener who had worked for the late Riddle family, and alerts Voldemort to Bryce's presence. During the fourth year Harry spends at Hogwarts, Voldemort's temporary body is sustained by Nagini's venom, harvested by Peter Pettigrew.In Harry Potter and the Order of the Phoenix, Harry takes a direct viewpoint of Nagini's attack on Arthur Weasley in one of his dreams, feeling that he (Harry) himself is the snake. Albus Dumbledore believes this to be due to Harry's special connection to Voldemort, with Harry's witnessing the attack by virtue of the fact that Voldemort's mind "happened to be" in Nagini at the time. This is the first indication of Nagini and Voldemort's deeper connection, having the ability to share each other's thoughts, and share them with Harry as well.
In the final book, Nagini consumes Charity Burbage, a Hogwarts Muggle Studies professor, after the Killing Curse is used on her. Nagini is later placed inside the body of Bathilda Bagshot by Voldemort, and uses the hiding place to launch a surprise assault on Harry when he visits Godric's Hollow. Because snakes can sense heat and movement in a way humans can't, Nagini is able to detect Harry and Hermione even when they are under the Invisibility Cloak. After discovering that Harry is searching for his Horcruxes, Voldemort places Nagini into a protective magical cage to prevent her from being killed, but still uses her to slay Severus Snape by expanding the cage over and on top of him. When Harry is apparently killed by Voldemort, Nagini is released from the protective enchanctment and is draped around Voldemort's shoulders during the Death Eaters' victory march back to Hogwarts. After Neville Longbottom openly defies Voldemort, Voldemort punishes him by forcing the Sorting Hat on his head and setting it on fire. The Death Eaters are then attacked and a battle ensues, and Neville pulls Godric Gryffindor's sword from the Hat and beheads Nagini