Tom Riddle's Diary was a blank journal which Tom Marvolo Riddle transformed into a Horcrux. This event took place in 1943, during Riddle's 6th year at Hogwarts School of Witchcraft and Wizardry. After Voldemort's first defeat in 1981, the diary was passed to Lucius Malfoy, who kept the diary in secret before slipping it into Ginny Weasley's school supplies at Flourish and Blotts in 1992. The diary used its magical influence to force Ginny to re-open the Chamber of Secrets, but it was destroyed by Harry Potter in 1993, with a Basilisk's fang.
History
Creation of the Diary as a Horcrux
When Tom Marvolo Riddle was attending his 6th 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 terrorize the school. Using the Basilisk, he murdered a student named Myrtle, who would later haunt the school as a ghost named Moaning Myrtle. Riddle used this murder to infuse the journal with his soul, transforming it into a Horcrux.
Eventually, Albus Dumbledore began suspecting Riddle. Riddle knew it was no longer safe to open the Chamber while he was still at Hogwarts. He created the diary so the Chamber could be opened again.
After the murder of Myrtle, Riddle questioned Professor Horace Slughorn about the properties of a Horcrux and how one might create multiples. Influenced by Riddle's charisma and believing his own wisdom was being admired, Slughorn told Riddle what little he knew on the subject.
Powers
As a Horcrux, the diary allowed a writer to communicate with the memory of the 16-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 siphon the life force from a reader and transfer it to Riddle's stored memory. This act was an effort to create a physical body for the 16-year-old soul of Tom Riddle. It is unclear whether other Horcruxes include any these same abilities, however, Salazar Slytherin's locket had a negative effect on the physical and emotional bodies of those who wore it.
Re-opening the Chamber (1992)
In 1992, Ginny Weasley came into the possession of the diary through the scheming of Lucius Malfoy. She soon learned how to use the diary to communicate with Tom Marvolo Riddle, though she was unaware of who Tom was, or if he was even real. Ginny wrote all of her deepest feelings and secrets in the diary, including her feelings towards Harry Potter. The diary was able to gain partial control of Ginny's mind, and forced her to re-open the Chamber of Secrets. Ginny was forced to not only vandalize the school corridors with terrifying threats in red paint, she also released the Basilisk. 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; they were petrified.
Harry began to search for the answers to the mysterious attacks. He heard strange voices in the walls of Hogwarts, which implied 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 waterlogged diary, which Ginny had discarded in an emotional fury. 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 his pet Acromantula, Aragog, as a scapegoat so Hogwarts would not be closed down.
Later, Ginny stole the diary from among Harry's belongings, and shortly thereafter went missing. She was forced to paint a threat on her life on the walls of a hallway and sealed herself inside the Chamber. Hogwarts was again under threat of being closed.
Destruction
Harry returned to the girls' lavatory and successfully discovered and opened the Chamber of Secrets. Harry, Ron Weasley, 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 Lockhart, Obliviating his memory. Harry continued into the chamber, where he discovered an Ginny unconscious 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 the 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.
Lord Voldemort was unaware that this Horcrux had been destroyed until he forced the truth out of Lucius Malfoy shortly after his return to power.
Behind the scenes
- In Harry Potter and the Chamber of Secrets (film), the same font, with a handwritten look, is used for Tom Riddle's handwriting in his diary and Hermione's note on the library page about the Basilisk, indicating "pipes."