The diary of Tom Riddle was a small, thin book with a black leather cover. It was purchased by a young Tom Riddle from a Muggle newsagent's shop on Vauxhall Road in London. Stamped on the front in faded gold lettering was the year “1943”. Internally, the diary's pages were completely blank and did not respond to being written on with ordinary ink. When a user wrote on a page with a quill, the ink would shine for a moment and then vanish as if absorbed into the paper, prompting a written response from the enchanted memory of Tom Riddle stored within.
As Voldemort's first Horcrux, the diary contained a fragment of his sixteen-year-old soul. Its primary purpose was to preserve that soul fragment, tethering Voldemort to life. However, it was also enchanted with powerful and insidious Dark Magic to act as a weapon.
In 1943, during his fifth year at Hogwarts, Tom Riddle used the Basilisk to murder a fellow student, Myrtle Warren, in a girls' bathroom. He used this murder to perform the complex Dark Magic required to split his soul and create his first Horcrux, embedding a fragment of his soul into the diary. Years later, before his first fall from power, Lord Voldemort entrusted the diary to his loyal Death Eater, Lucius Malfoy, for safekeeping. He instructed Malfoy to ensure the diary was one day smuggled back into Hogwarts to reopen the Chamber of Secrets, though he did not reveal its true nature as a Horcrux. In the summer of 1992, during a confrontation at Flourish and Blotts, Lucius Malfoy slipped the diary into Ginny Weasley's cauldron among her secondhand schoolbooks. Over the following school year, Ginny confided in the diary and became possessed by Riddle's memory. Under its influence, she unleashed the Basilisk, which petrified several students. Frightened, Ginny attempted to dispose of the diary by flushing it down a toilet in what was now Moaning Myrtle's bathroom. Harry Potter later found the diary. After he communicated with it and was shown the false memory, Ginny stole it back, fearing Harry would discover her role in the attacks. The diary then forced her into the Chamber of Secrets to complete its plan of resurrection. Harry followed and, with help from Fawkes the phoenix and the Sword of Godric Gryffindor, slew the Basilisk. He then used a broken Basilisk fang to stab the diary, destroying the Horcrux within it. The highly corrosive Basilisk venom is one of the few substances capable of destroying a Horcrux.
The diary is the central mystery and antagonistic force in Harry Potter and the Chamber of Secrets. Its discovery and eventual destruction by Harry Potter provides the first major clue to Voldemort's past and the nature of his immortality. It represents Harry's first direct confrontation with a piece of Voldemort's soul. In Harry Potter and the Half-Blood Prince, Albus Dumbledore reveals the diary's true nature as a Horcrux to Harry. This revelation reframes Harry's past victory as a critical, if then unknown, step towards defeating Voldemort. The diary serves as the primary example that helps Harry understand the Horcrux concept, setting him on the path to hunt and destroy the remaining ones.