Tom Riddle's diary

"The diary. My diary. Little Ginny's been writing in it for months and months, telling me all her pitiful worries and woes — how her brothers tease her, how she had to come to school with secondhand robes and books, how — how she didn't think famous, good, great Harry Potter would ever like her… It's very boring, having to listen to the silly little troubles of an eleven-year-old girl. But I was patient. I wrote back. I was sympathetic, I was kind. Ginny simply loved me. "No one's ever understood me like you, Tom… I'm so glad I've got this diary to confide in… It's like having a friend I can carry around in my pocket…" […] So Ginny poured out her soul to me, and her soul happened to be exactly what I wanted… I grew stronger and stronger on a diet of her deepest fears, her darkest secrets."

- Tom Riddle to Harry Potter, referring to Ginny Weasley

Tom Riddle's diary was a blank diary which Tom Riddle transformed into a Horcrux. This event took place in 1943, during Riddle's fifth year at Hogwarts School of Witchcraft and Wizardry, and it was the first Horcrux he ever made. Before Lord Voldemort's first defeat in 1981, the diary was passed to Death Eater Lucius Malfoy, who kept the diary, in secret, before slipping it into Ginny Weasley's cauldron of school supplies at Flourish and Blotts in 1992. The diary used its magical influence to possess and force Ginny to re-open the Chamber of Secrets, but it was destroyed by Harry Potter in 1993 with a basilisk's fang.

Creation of the diary as a Horcrux
"The diary was proof that he was the heir of Slytherin; I am sure that Voldemort considered it of stupendous importance."

- Albus Dumbledore on the diary's importance to Voldemort.

When Tom Marvolo Riddle was in his fifth year at Hogwarts, he used his ability to speak Parseltongue to open Salazar Slytherin's Chamber of Secrets. He further used this language ability to order the Chamber's basilisk to terrorise the school and hunt down its Muggle-born students. Eventually one, a Ravenclaw girl named Myrtle, was killed. Riddle used this murder to infuse the journal with a piece of his soul, transforming it into a Horcrux.

After the death of Myrtle, Professor Dumbledore, then professor of Transfiguration, began suspecting Riddle. Knowing it was no longer safe to open the Chamber, and that the school might be shut down if another student was harmed, Riddle framed Rubeus Hagrid and his pet Aragog for the crimes. He imbued the diary with other powers, so that the Chamber could be opened again in the future.

In the following years, Riddle also questioned Potions professor Horace Slughorn about the properties of a Horcrux and how one might create multiple ones. Influenced by Riddle's charisma and believing his own wisdom was being admired, Slughorn told Riddle what little he knew on the subject.

Re-opening of the Chamber of Secrets
". . . I was patient. I wrote back. I was sympathetic, I was kind. Ginny simply loved me . . . No one's ever understood me like you, Tom . . . I'm so glad I've got this diary to confide in . . . It's like a friend I can carry around in my pocket . . . If I say it myself, Harry, I’ve always been able to charm the people I needed."

- Tom Riddle to Harry Potter on his manipulation of Ginny Weasley through the diary

Some time during the First Wizarding War, Lord Voldemort entrusted the diary to the Death Eater Lucius Malfoy. The plan was to use the diary to reopen the Chamber, but Voldemort fell before this plan came to fruition. However, Malfoy still possessed the diary and, in 1992, he planted it on Ginny Weasley, hoping to kill three birds with one stone by sabotaging Ginny's father, purging the school of muggle-borns, and eliminating an incriminating piece of dark artefact. When the Weasleys and Harry left to King's Cross Station, Ginny accidentally left the diary at the Burrow. Just before they got to the motorway, she remembered the diary and demanded to be allowed to go back and get it. She soon learned how to use the diary to communicate with Tom Riddle, though she was unaware that he was Lord Voldemort, or if he was even real. Ginny wrote all of her deepest feelings and secrets in the diary, including her feelings of infatuation towards Harry Potter.

Her emotional vulnerability allowed the fragment of Voldemort's soul within the diary to gain partial control of her mind and force her to re-open the Chamber of Secrets. Ginny was forced not only to vandalise the school corridors with terrifying threats written in chicken blood, but also to release the basilisk within the Chamber and strangling chickens to prevent the snake from dying, all the while in a sort of trance, never knowing what she had been doing. The basilisk attacked several members of the Hogwarts community, including Mrs. Norris, Colin Creevey, Justin Finch-Fletchley, Nearly-Headless Nick, Hermione Granger, and Penelope Clearwater. These individuals were not murdered by the basilisk, but petrified - a result of not seeing the basilisk directly.

When the attacks began, Harry Potter, Ron Weasley, and Hermione Granger began to investigate the matter. They initially suspected Draco Malfoy, who was openly prejudiced against Muggle-borns, but after using Polyjuice Potion to spy on him, they discovered that he knew as little about the attacks as they did. Hermione eventually deduced that the monster had to be a basilisk, but she was petrified before she could inform Harry or Ron, though she managed to leave them a relatively straight-forward clue.

Harry heard strange voices in the walls of Hogwarts, which exhorted the urge to kill. Harry followed the voices through the Hogwarts corridors until he came upon the second floor girls' lavatory. It was here that he discovered the water-logged diary, which Ginny had discarded when she suspected that it was she who was petrifying the Muggle-borns, and that she finally realised that the diary was the cause. Soon enough, Harry was communicating with Tom Riddle, who showed him a memory within the diary, in which Riddle confronted Rubeus Hagrid about the keeping of a dangerous pet. This implied to Harry that Hagrid had opened the Chamber of Secrets, but Riddle was merely using Hagrid's pet Acromantula, Aragog, as a scapegoat so Hogwarts would not be closed down.

Later, Ginny, having seen the diary amongst Harry's books, stole the diary from among Harry's room, fearing that Riddle would tell Harry about her feelings towards him, and shortly thereafter went missing. She was forced to paint a threat against her life on the walls of a hallway and sealed herself inside the Chamber. Hogwarts was again under threat of being closed down.

Destruction
"Haven't I taught you anything? What have I always told you? Never trust anything that can think for itself if you can't see where it keeps its brain!"

- Arthur Weasley to his daughter after her rescue

Harry returned to the girls' lavatory and successfully discovered and opened the Chamber of Secrets. He, Ron, and Professor Gilderoy Lockhart made their way into the Chamber. Lockhart, who had no plans to actually fight the basilisk, attacked the students with Ron's own broken wand, which backfired on him, wiping his memory. Harry continued into the Chamber, where he discovered an unconscious Ginny next to Tom Riddle, who was gaining strength and form. Riddle revealed the truth of his identity and plans to Harry and unleashed the basilisk. Harry defeated the basilisk with the assistance of Dumbledore's phoenix Fawkes and Godric Gryffindor's Sword. Harry then took one of the basilisk's fangs and stabbed the diary with it. This successfully destroyed the Horcrux, as basilisk venom is one of the few substances with that ability.

Lord Voldemort was unaware that this Horcrux had been destroyed, until he forced the truth out of Lucius Malfoy (who did not know the diary was a Horcrux) shortly after his return to power in June of 1995, due to the portion of the soul being separated from his body for such a long time. Despite the known risks of using the diary as a weapon instead of a safeguard, Voldemort was nevertheless furious to learn of its destruction due to Lucius implementing the plan without Voldemort's say-so, in order to dispose of the diary for his own benefit, in attempting to destroy the Weasley family and Dumbledore's reputation, as well as getting rid of an incriminating Dark Art artefact (the Ministry was conducting a search at the time); this was one of the many reasons he lost trust in the Malfoy family and why Lucius was punished and demoted.

Although Voldemort was unaware of the diary's destruction until Malfoy reported it to him, he attributed this to his non-corporeal status at the time of its loss; as a result, he took no additional precautions to safeguard the remaining horcruxes, assuming that they were safe and undamaged until the break-in to Gringotts confirmed that someone else was hunting for them.

Powers
"While the magical container is still intact, the bit of soul inside it can flit in and out of someone if they get too close to the object. I don’t mean holding it for too long…I mean close emotionally. Ginny poured her heart out into that diary, she made herself incredibly vulnerable. You’re in trouble if you get too fond of or dependent on the Horcrux."

- Hermione Granger on a Horcrux's ability to influence a person

As a Horcrux, the diary allowed a writer to communicate with the memory of the sixteen-year-old Tom Riddle, merely through writing on the journal's blank pages. The diary could transport the reader into a realm of memories, much like the Pensieve. The diary was also able to siphon the life force from a reader and transfer it to Riddle's stored memory. This act was an endeavour to create a physical body for the sixteen-year-old soul of Tom Riddle. The closer a writer became to the memory of Tom Riddle emotionally, the more power the diary would acquire over him or her. As this diary contained Riddle's soul, it also housed his magical powers, including the ability to speak Parseltongue, even by possessing the writer to speak it, which was instrumental in reopening the Chamber of Secrets.

It is unclear whether other Horcruxes include any of these same abilities; however, Salazar Slytherin's Locket had a negative physical and emotional effect on those who wore it, and Cadmus Peverell's ring fatally cursed Albus Dumbledore. Also, it is unknown if Riddle placed powerful enchantments to protect the diary from being destroyed by conventional means, as he did with all of his other Horcruxes, but it is highly likely that this was indeed the case.

Behind the scenes

 * The diary was originally purchased from a Muggle shop on Vauxhall Road in London.
 * Considering Draco Malfoy's reaction to finding the diary with Harry, Lucius apparently never showed the book to his son.
 * This Horcrux revealed to Harry Potter that Lord Voldemort was once Tom Riddle.
 * In an interview with J. K. Rowling, she states that "My sister used to commit her innermost thoughts to her diary. Her great fear was that someone would read it. That's how the idea came to me of a diary that is itself against you. You would be confiding everything to pages that aren't inanimate."
 * J.K. Rowling has also stated, "Now, the diary to me is a very scary object, a really, really frightening object. This manipulative little book, the temptation particularly for a young girl to pour out her heart to a diary, which is never something I was prone to, but my sister was. The power of something that answers you back, and at the time that I wrote that I'd never been in an Internet chat room. But I've since thought "Well it's very similar." Just typing your deepest thoughts into the ether and getting answers back, and you don't know who is answering you. And so that was always a very scary image to me, in the book, and I thought it worked very well in the film. You could understand when he started writing to see these things coming back to him, and the power of that, that secret friend in your pocket."
 * In the second book and video game adaptation of it, Harry stuffed the diary in his sock. In the film adaptation, Harry puts the sock in the diary.
 * In the book and film versions of Harry Potter and the Chamber of Secrets, Harry destroys the diary with the basilisk's fang. In the GameBoy Advance version of the game, Harry stabs it with Godric Gryffindor's Sword. In the PC and PS2 version of the game the Basilisk lands on the diary.
 * According to Dumbledore, Voldemort tends to use extremely valuable trophies or sentimentally important artefacts to create his Horcruxes, and that the reason for this diary was chosen was due to it proving him to be the heir of Slytherin. Otherwise, the fact that the diary was a common artefact made it very ironic to be considered valuable in Voldemort's eyes, especially when it was manufactured by his hated enemies: Muggles.
 * The diary is the only of Voldemort's artificial Horcruxes that he acquired legally.
 * This diary was used as a weapon, to unleash the basilisk once again in Hogwarts, rather than keeping the portion of the soul safe, which is what Horcruxes were meant for. It shares this attribute with a fellow Horcrux of Voldemort, Nagini, who was sent on missions from time to time.
 * In LEGO Harry Potter: Years 1-4, a Year 2 level had the same name as the book.
 * The Noble Collection has released a replica of the diary.
 * The diary is Bonnie Wright's favourite prop from the films.
 * The diary had a small hole on the front of it and a dent coming out the back at the end of Chamber Of Secrets, but in The Half-Blood Prince, it somehow has a huge gaping hole in that is not even the size of a fang, creating a potential consistency error, although, perhaps, the Basilisk venom was of an adequate toxicity to continue inflicting damage on the book for some time after the fang was removed.
 * In the original script for the film of Harry Potter and the Half-Blood Prince, Ginny states that she hid the diary in the Room of Hidden Things, which she learned about from Fred and George Weasley, and says that she wishes that she had just left it there. Even if this line had been included in the final film, however, it cannot be considered canon, as in the books Fred and George Weasley only knew of the room as a broom-cupboard they once hid in and were never aware that it could be used at any time until the revelation of the room and Dumbledore's Army's use of it in Harry Potter and the Order of the Phoenix.
 * In LEGO Harry Potter: Years 1-4,Ginny Weasley plays with the book ignoring Harry by hitting him in the ribs.

Appearances

 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter: A Pop-Up Book
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7
 * Pottermore
 * Harry Potter for Kinect