Lord Voldemort

Lord Voldemort makes his debut in Harry Potter and the Philosopher's Stone. In this story, Rowling introduces him as the Dark Lord who murdered Harry's parents, but as a result of his mother's love and willingness to sacrifice herself for him, baby Harry survives when Voldemort tries to murder him with a Killing Curse. Voldemort is disembodied, and Harry carries a mysterious scar on his forehead as a result. During the course of the book, Voldemort unsuccessfully tries to regain his dissolved body by stealing the titular Philosopher's Stone. To achieve his objective, Voldemort uses Professor Quirrell's aid by latching himself onto the back of Quirrell's head. However, at the climax of the book, Harry manages to prevent Voldemort from stealing the stone.

In the second instalment, Harry Potter and the Chamber of Secrets, Rowling introduces the character of Tom Marvolo Riddle, a manifestation of a teenage Voldemort that resides inside a magical diary found by Ginny Weasley. In this book, Ginny is written as a shy girl with a crush on Harry. Feeling anxious and lonely, she begins to write into the diary and shares her deepest fears with the sympathetic Tom. However, at the climax of the story, when Tom Marvolo Riddle rearranges the letters in his name to create the anagram of "I am Lord Voldemort", the nature of Tom as a magical manifestation of the boy who would later grow up to become the Dark Lord is revealed. Riddle states he has grown strong on her fears and eventually possesses Ginny. He then uses her as a pawn to unlock the Chamber of Secrets, whence a basilisk is set free and petrifies several Hogwarts students. Harry defeats both the Riddle from the diary and the basilisk. In Harry Potter and the Half-Blood Prince, Dumbledore reveals to Harry that the diary was one of Voldemort's Horcruxes (an external vessel which contains a part of his torn soul).

In the third book, Harry Potter and the Prisoner of Azkaban, Voldemort does not appear, either in person or in the form of a magical manifestation. He is, however, heard when Harry passes out from the harsh effects of a Dementor. Towards the end of the story Sybill Trelawney, the Divination professor, makes a rare genuine prophecy: The Dark Lord lies alone and friendless, abandoned by his followers. His servant has been chained these twelve years. Tonight, before midnight, the servant will break free and set out to rejoin his master. The Dark Lord will rise again with his servant's aid, greater and more terrible than ever before. Tonight... before midnight... the servant... will set out... to rejoin... his master... Though it is initially implied that the prophecy refers to Sirius Black, the book's ostensible antagonist, the servant is eventually revealed to be Peter Pettigrew, who, since the fall of Voldemort, has been disguised as Ron Weasley's pet rat, Scabbers.
In the fourth instalment of the series, Harry Potter and the Goblet of Fire, Voldemort appears at the start and the climax of the book. Rowling lets many seemingly unrelated plot elements fall into order. It is revealed that Voldemort's minion Barty Crouch Jr, disguised as Hogwarts professor Mad-Eye Moody, has manipulated the events of the Triwizard Tournament. Voldemort's goal is to teleport Harry as a reluctant participant to the Little Hangleton graveyard, where the Riddle family is buried. Harry is captured and, after Pettigrew uses Harry's blood to fulfil a gruesome magical ritual, Voldemort regains his body and is restored to his full power. For the first time in the series, Rowling describes his appearance: "tall and skeletally thin", with a face "whiter than a skull, with wide, livid scarlet eyes and a nose that was as flat as a snake’s with slits for nostrils".Rowling writes that his "hands were like large, pale spiders; his long white fingers caressed his own chest, his arms, his face; the red eyes, whose pupils were slits, like a cat's, gleamed still more brightly through the darkness". It was revealed that, while in Albania, Pettigrew had captured the Ministry of Magic official Bertha Jorkins, who was tortured for information about the Ministry. After they learned that Barty Crouch Jr, a faithful Death Eater, had been smuggled out of Azkaban and was privately confined at his father's house, they killed her. With Pettigrew's help, Voldemort created a small, rudimentary body, corporeal enough to travel and perform magic, and formulated a plan to restore his own body by capturing Harry. A portion of the plan had been overheard by Frank Bryce, a gardener, whom Voldemort then killed. Voldemort then completes his plan and returns to life in his full body as a result of the ritual with Harry's blood. He then summons his Death Eaters to the graveyard to witness the death of Harry as he challenges Harry to a duel. However, when Voldemort duels Harry, their wands become magically locked together due to the twin Phoenix feather cores of the wands. Because of a phenomenon later revealed as Priori Incantatem, ghost-like manifestations of Voldemort's most recent victims (including Harry's parents) then appear and distract Voldemort, allowing Harry just enough time to escape via Portkey with the body of fellow-student, Cedric Diggory, who was murdered by Pettigrew on Voldemort's orders.

In the fifth book, Harry Potter and the Order of the Phoenix, Voldemort appears at the climax, having again carefully plotted against Harry. In this book, Harry goes through extreme emotional stress, and according to Rowling, it was necessary to prove that Harry is emotionally vulnerable and thus human, in contrast to his nemesis Voldemort, who is emotionally invulnerable and thus inhuman: "[Harry is] a very human hero, and this is, obviously, there’s a contrast, between him, as a very human hero, and Voldemort, who has deliberately dehumanised himself. […] and Harry, therefore, did have to reach a point where he did almost break down." In this book, Voldemort makes liberal use of the Ministry of Magic's refusal to believe that he has returned. Voldemort engineers a plot to free Bellatrix Lestrange and some other Death Eaters from Azkaban and then embarks on a scheme to retrieve the full record of a prophecy regarding Harry and himself which is stored in the Department of Mysteries. He sends a group of Death Eaters to retrieve the prophecy, where they are met by the Order of the Phoenix. All but Bellatrix are captured, and Voldemort engages in a ferocious duel with Dumbledore. Voldemort attempts to possess Harry but finds that he cannot; Harry is too full of that which Voldemort finds detestable: love. Sensing that Dumbledore could win, Voldemort disapparates, but not before the Minister for Magic sees him in person, making his return to life public knowledge.
Hero Fiennes-Tiffin as child Tom Riddle in Harry Potter and the Half-Blood Prince

In the sixth book, Harry Potter and the Half-Blood Prince, Voldemort once again declares war, and begins to rise to power once more. He murders Amelia Bones of the Department of Magical Law Enforcement, and begins to target members of the Order of the Phoenix, including Emmeline Vance.

Rowling uses several chapters as exposition to establish Voldemort's backstory. In a series of flashbacks, using the pensieve as a plot device, she reveals that Voldemort is the son of the witch Merope Gaunt and Muggle Tom Riddle, Sr. Riddle abandons Merope before their child's birth, soon after which Merope dies, just hours after giving birth. Tom Riddle, Sr. never attempts to find his son. After living in an orphanage, young Tom is told that he is a wizard by Albus Dumbledore, who arranges for him to attend Hogwarts. Riddle is outwardly a model pupil, but is in reality a sadist who enjoys using his powers to harm and control people. He eventually murders his father and grandparents as revenge for abandoning him. The book also discusses Riddle's hatred of "Muggles" (non-magical humans), his obsession with Horcruxes, and his desire to split his soul in order to achieve immortality. Rowling stated Voldemort's conception under the influence of a love potion is related to his inability to understand love.

In the main plot of the book, Voldemort's next step is to engineer an assault on Hogwarts, and to attack Dumbledore himself. This is accomplished by Draco Malfoy, who arranges transportation of Death Eaters into Hogwarts by means of a pair of Vanishing Cabinets, which bypass the extensive protective enchantments placed around the school. The cabinets allow Voldemort's Death Eaters to enter Hogwarts, where battle commences and Dumbledore is cornered. Hogwarts professor (and triple agent) Severus Snape uses the Killing Curse against Dumbledore when Draco is unable to do so himself.
In Harry Potter and the Deathly Hallows, Voldemort furthers his quest for ultimate power. He disposes of the Minister for Magic and replaces him with Pius Thicknesse, who is under the Imperius Curse. Establishing a totalitarian police state, he has Muggle-borns persecuted and arrested for "stealing magic" from the "pure blood" wizards. After failing to kill Harry with Lucius Malfoy's borrowed wand (to avoid the effect of Priori Incantatem), he goes on a murderous search for the Elder Wand, the most powerful wand ever created, seeing it as the weapon he needs to overcome Harry's wand and make him truly invincible. He goes on a quest that takes him out of the country to Gregorovitch's wand shop, where he kills the old wandmaker. His journey also takes him to Nurmengard, the prison where Gellert Grindelwald is kept, and he kills Grindelwald as well. He finally locates the Elder Wand and steals it from Dumbledore's tomb.

Later, he finds out that Harry and his friends are stealing and destroying his Horcruxes. After offering the occupants of Hogwarts mercy if they give up Harry, he assembles a large army and launches an invasion of the castle, where Harry is searching for Ravenclaw's Lost Diadem, one of the Horcruxes. Voldemort orders his pet snake Nagini to execute Snape, believing it would make him the true master of the Elder Wand, since Snape killed Dumbledore. He then calls an hour's armistice, in exchange for Harry. When Harry willingly walks into Voldemort's camp in the Forbidden Forest, Voldemort strikes him down with the Elder Wand. However, the use of Harry's blood to resurrect Voldemort's body proves to be a major setback: while Harry's blood runs in Voldemort's veins, Harry cannot be killed as his mother's protection lives on now in Voldemort too. Instead, Voldemort himself destroys the part of his own soul that resides in Harry’s body. Voldemort forces Rubeus Hagrid to carry the apparently lifeless body of Harry back to the castle as a trophy, sparking another battle during which Nagini, his last Horcrux, is destroyed by Neville Longbottom. The battle then moves into the Great Hall, where Voldemort fights Minerva McGonagall, Kingsley Shacklebolt, and Horace Slughorn simultaneously. Harry then reveals himself and explains to Voldemort that Draco became the true master of the Elder Wand when he disarmed Dumbledore; Harry, in turn, won the wand's allegiance when he took Draco's wand. Voldemort nonetheless casts the Killing Curse with the Elder Wand while Harry uses a Disarming Charm with Draco's, but the Elder Wand refuses to kill its master and the spell rebounds on Voldemort who, with all of his Horcruxes destroyed, is finally killed.

Rowling stated that after his death, Voldemort is forced to exist in the stunted infant-like form that Harry sees in the King's Cross-like Limbo after his confrontation with Voldemort in the Forbidden Forest. Rowling also mentioned that, despite his extreme fear of death, he is unable to become a ghost.