Nagini is Voldemort's pet snake, and a minor antagonist in the Harry Potter films, but is the secondary antagonist of the final film, Harry Potter and the Deathly Hallows: Part 2.
Not much was known about Nagini's early life or when she was acquired by Voldemort. It was unknown whether Voldemort owned Nagini before his fall in 1981, or if he had found her during his exile in Albania. However, it was stated that in the woods of Albania, the phantom-like form that was the remnants of Voldemort had befriended many animals, yet only snakes could bear having him inhabit them, thus leading to the notion that Voldemort and Nagini's relationship had originally commenced in Albania.
Voldemort used Nagini to help him regain strength and a basic physical form after living in a weak, spectral state, following his attempt to kill an infant Harry Potter, during which he was struck by his own rebounding Killing Curse. He had the servant who returned to him, Peter Pettigrew, milk her venom, and with it, create a potion that sustained him with a temporary body until he could regain his true form. Nagini further kept Voldemort alive after he turned her into a Horcrux, with the murder of Bertha Jorkins, in 1994; he was as attached to her as he could of any living creatures, and her underlining of his heritage to the legendary Salazar Slytherin only furthered her mystique to him. His habit of keeping her unusually close to him was what led Albus Dumbledore to suspect that she had become a Horcrux for quite sometime.
In the summer of 1994, Nagini, along with Voldemort and Pettigrew, sought shelter in the abandoned Riddle House until they could instigate the plan to capture Harry Potter. Nagini alerted Voldemort to the presence of an Frank Bryce, an old Muggle gardener who had worked for the late Riddle family, who had overheard a discussion between Pettigrew and Voldemort. Soon after, Nagini slithered past Bryce and alerted Voldemort, and the Dark Lord murdered the Muggle man.
Upon Voldemort's rebirth via the capture of Harry, Nagini was slithering around the Little Hangleton graveyard that the ceremony took place, and was promised that she could feed on the boy's corpse after her master had killed him. She was deprived of this promise when Harry escaped.
Shortly before the Christmas of 1995, Nagini was presumably sent by the Dark Lord to retrieve a Prophecy in the Ministry of Magic. After she had infiltrated the area, Nagini met up with Order of the Phoenix member, Arthur Weasley, who was sent to guard the very same prophecy. Nagini, unable to resist, and who had attracted Arthur's attention already, attacked him. The attack was witnessed by Harry in one of his dreams, and felt that he (Harry) was the snake. Critically injured, Arthur was taken to St Mungo's. He survived the attack, but was required to take a Blood-Replenishing Potion at regular intervals. At the suggestion of Trainee Healer, Augustus Pye, Arthur tried stitches, a Muggle replacement, of which his wife, Molly, disapproved.
Nagini's venom seemed to delay blood clotting, and even dissolved the stitches, which caused the wounds to bleed profusely when the bandages were removed. Ultimately, an antidote to the poison was found and he was cured. The reason that Albus Dumbledore believed Harry saw the vision as if he was the snake was 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 was the first indication of Nagini and Voldemort’s deeper connection, having the ability to share thoughts and connect with Harry.
In 1997, Nagini was present at the Meeting at Malfoy Manor. When the Death Eaters were jeering at Charity Burbage and the Malfoy family's humiliation, because of a disgraceful marriage in the family, the great snake got agitated, disliked the noise, opened its mouth wide, and hissed angrily, but to deaf ears. Voldemort, after stroking the angry snake, called for silence.
Near the end of the meeting, Voldemort's prisoner, Hogwarts Muggle Studies teacher, Charity Burbage, was murdered by the Dark Lord by means of the Killing Curse and fed to Nagini.
Later, when Thorfinn Rowle and Antonin Dolohov failed to capture Harry after the Luchino Caffe Duel, Voldemort offered them a choice in punishment: being further Tortured by Draco Malfoy or to be killed and devoured by Nagini. The two apparently chose further torture, as both survived for the next battle.
Nagini was later placed inside the corpse of Bathilda Bagshot by Voldemort's use of extremely Dark Magic and stationed in Godric's Hollow to wait for Harry Potter. When Harry and Hermione Granger arrived there, she lured them into the late witch's home, signalled Voldemort due to the special connection that the two of them shared (Voldemort seemed to have an excessive amount of control over the snake even for a Parselmouth), and then launched a surprise assault on Harry, while the latter was investigating a pile of dirty laundry on a dresser.
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. However, Hermione came to Harry's assistance, using a Blasting Curse against the snake; the two then jumped out the window and Disapparated just before Voldemort arrived. Later, on Potterwatch, Lee Jordan revealed that the remains of Bathilda's body had been found after being informed by the Order of the Phoenix.
After he discovered that Harry was searching for his Horcruxes in 1998, Voldemort placed Nagini into a Protective Magical Cage to prevent her from being killed. Voldemort told her that it was for her own good, though since he was trying to ensure his own immortality, it was primarily for his. From that point forward, Voldemort decided that it was no longer safe to send Nagini on any more missions for him.
Shortly before the one-hour armistice during the Battle of Hogwarts, Voldemort used Nagini to murder Severus Snape in the Shrieking Shack by expanding the cage over and on top of him. Nagini proceeded to sink her fangs into Snape's neck, and when Voldemort pulled the protective cage off of Snape, he fell to the ground with blood gushing from the wound in his neck.
When Harry was apparently killed by Voldemort, Nagini was released from the protective enchantment as he believed there to be no more threats to her life, and she was draped around Voldemort's shoulders during the Death Eaters’ victory march back to Hogwarts.
After Neville Longbottom had openly defied him, Voldemort punished him by forcing the Sorting Hat onto his head and setting it on fire. The Death Eaters were then attacked, and during the ensuing battle, Neville pulled out Godric Gryffindor's Sword from the Sorting Hat (as Harry had done during the Skirmish in the Chamber of Secrets), and, after he swung the blade in one upward stroke, beheaded Nagini, whose head spun high into the air as her body slumped onto the ground, while Voldemort screamed in rage.
In an alternate reality created by Albus Potter and Scorpius Malfoy, by saving Cedric Diggory which caused him much humiliation during the Triwizard Tournament, they turned him into a bitter man who would join the Death Eaters. As a result, Cedric killed Neville during the Battle of Hogwarts, thus preventing him from killing Nagini. This allowed Voldemort to retain his immortality, and thus kill Harry, which led to the Dark Lord to effectively conquer the world.
Nagini was a large, green snake, roughly twelve feet long and as thick as a man's thigh.
Personality and traits
Nagini was completely loyal to Lord Voldemort. While she remained aloof, if not malevolent, to most others, she obeyed Voldemort completely, and seemed to enjoy being close to him, as she often slithered up to his shoulders to receive his affection.
She was also shown to be very intelligent and was able to understand the concept of strategies and plans, as seen when she set up a trap for Harry, due to her possessing a portion of Voldemort's soul. She was rather cunning and deceptive, and seemed to have understood how humans behaved.
Magical abilities and skills
Nagini was extremely deadly in combat, just like her master. She was very fast, capable of striking her enemies with great dexterity and speed, and was able to hold her own against wizards and witches on several occasions. Nagini was also strong and was able to hold Harry Potter in place by wrapping him up in her thick, strong coils when she attacked him in Godric's Hollow. If not for Hermione's Blasting Curse, Nagini's physical strength would have allowed her to hold Harry in her coils until Voldemort had arrived.
Nagini was a venomous snake and a single bite could result in a wound that was slow to heal (the extent to which this was due to her status as a Horcrux was unclear). Though her venom was not always fatal, it made it difficult for wounds to close and Arthur Weasley's injuries continued to bleed long after Nagini had bitten him. If bitten at a vital location, it can result in fatal blood loss within a short amount of time, as Severus Snape died at a bite to the neck.
Bond with Voldemort
Being one of Voldemort's Horcruxes, Nagini had a strong telepathic bond with her master and was capable of speaking to him over long distances. Voldemort could also possess Nagini at some circumstances.
As a Horcrux
Nagini contained a fragment of Voldemort's soul.
Voldemort had a special relationship with Nagini, as she was his pet and one of his Horcruxes. According to Dumbledore, Voldemort had strong feelings for Nagini that he had not for anyone else; she was the one living thing that he had ever cared about. He was able to communicate with Nagini as a Parselmouth, and he turned her into a Horcrux in 1994, after he murdered Bertha Jorkins, which accounted for the unusual amount of control that he had over her. By 1997, and likely before then, he was able to communicate with her silently and across great distances, as she was able to signal to him when Harry Potter arrived at Godric's Hollow. This may have been due to her nature as a Horcrux, which gave her a link to Voldemort like the one Harry possessed.
Voldemort was also able to possess Nagini, which he did in December 1995, during the attack on Arthur Weasley. In 1994, Peter Pettigrew milked Nagini's venom to return Voldemort to a rudimentary body, and as a Horcrux, her very existence contributed to maintaining Voldemort's immortality. Therefore, it's questionable if Voldemort actually cares about his snake or just the fragment of his soul inside her. Voldemort usually sent her on missions, such as infiltrating the Ministry, or holding Harry Potter in Godric's Hollow. He allowed her to consume the corpses of those he killed as a reward, like that of Charity Burbage, though this also served as a way for him to dispose of bodies. Voldemort apologised to Nagini when he could not feed Pettigrew to her, as Pettigrew's failure of keeping Crouch Snr was rectified, and therefore, allowed him to escape the death penalty and not become Nagini's meal. Unlike many of his subordinates, Voldemort did not punish Nagini for failures, as when she failed to retrieve the prophecy or let Harry Potter escape from her grasp.
Voldemort stopped sending Nagini on missions after he realised that Harry knew about the existence of his Horcruxes and was tracking them down. Voldemort was enraged when Nagini was beheaded by Neville Longbottom with Godric Gryffindor's Sword, and he attempted to kill him in retaliation. Nagini was the last Horcrux to be destroyed, and her death led to Voldemort's final defeat and death.
Nagini was aloof and indifferent to the Death Eaters as a whole, but sometimes could be irritated by them, as she hissed angrily at their loud laughter at the Malfoy family, to which none of them took notice until Voldemort ordered their silence for her sake. She allowed Peter Pettigrew to milk the venom out of her fangs in order for it to be used for their master, but otherwise would have been glad to devour any Death Eaters that her master permitted, as a form of punishment for their failure; Pettigrew, Dolohov and Rowle were threatened to become Nagini's meal in lieu of torture. Because of this, many of the Death Eaters were terrified of Nagini for her deadly and ruthless behaviour.
Nagini was frequently used by Voldemort to kill others, or to dispose of their dead bodies. In 1995, he promised Nagini that she could devour Harry Potter after he had killed him. However, she was denied this when Harry escaped from the graveyard shortly after the promise was made. It was likely that Frank Bryce met that intended fate, sometime in 1994. Voldemort threatened Pettigrew with being fed to Nagini if Bartemius Crouch Snr was able to contact Albus Dumbledore about the plot behind the Triwizard Tournament. Nagini also attacked Arthur Weasley but he survived the bites after Harry had a vision on it and he was quickly aided. She's also known for disposing the body of Charity Burbage and for killing Severus Snape.