Mary GrandPré's illustration of Nagini in Harry Potter and the Goblet of Fire.Nagini (Hindi: nāginī, pronounced [ˈnɑːɡɪniː]; variant of Hindi: nāgin, a female cobra, from Hindi/Sanskrit: nāg, a cobra) is a serpent used by Voldemort to create his final Horcrux. Voldemort intended Nagini to be his sixth Horcrux, which would have split his soul into seven pieces, a number he considered to have magically powerful properties. Since he had inadvertently made Harry into a de facto Horcrux (a fact he had not known), Nagini was in fact his seventh Horcrux, although only six Horcruxes survived at any one time since the diary had already been destroyed. He cast the spells needed to create the Horcrux after murdering Ministry official Bertha Jorkins. As with his diary, Voldemort intended Nagini to be a tool as well as a safeguard to his immortality. Nagini is introduced in the first chapter of the Goblet of Fire. The snake is killed by Neville Longbottom in the final chapter of The Deathly Hallows.
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 some snakes can sense heat and movement in a way humans cannot, 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 enchantment 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.
Since she was Voldemort's last remaining Horcrux, her death sets up the series' climactic duel between Voldemort and Harry Potter. With no Horcruxes remaining and, therefore, rendered mortal, Voldemort's death in that duel is final.