Tom Riddle's Diary is a magical object. It was created by Tom Riddle, who would later be known as Lord Voldemort.
Background[]
Back when Lord Voldemort was a sixteen-year-old Hogwarts student named Tom Riddle, he opened the Chamber of Secrets. This nearly resulted in the school being forced to close, so Riddle, not wanting to return to the Muggle orphanage in which he grew up, framed Rubeus Hagrid for the attacks. Riddle used the murder of Moaning Myrtle to split his soul and turn his diary into a Horcrux, which he had been taught how to do by an unwitting Horace Slughorn. The diary now contained a fragment of Riddle's soul, which could force an unwilling victim to reopen the Chamber of Secrets. Many years later, Voldemort left the diary in the possession of one of his followers, Lucius Malfoy.
Appearances in the Films[]
At the start of Harry Potter and the Chamber of Secrets, Lucius slips the diary into Ginny Weasley's cauldron. Ginny doesn't realize it, but the diary takes possession of her and forces her to open the Chamber of Secrets. During the study hall scene, Ginny is briefly shown writing in what is presumably Riddle's diary.
Eventually, Ginny grows suspicious of the diary and tries to throw it away in Moaning Myrtle's bathroom. Myrtle mistakenly believes that the diary was thrown at her out of spite and floods the bathroom, attracting the attention of Harry Potter and Ronald Weasley. While experimenting with the diary, Harry discovers how to communicate with Riddle's spirit. Riddle shows him the night when he captured Hagrid, thinking that this would enable him to gain Harry's trust. Soon afterwards, however, Ginny steals the diary back from Harry's dormitory.
At the end of the year, Riddle brings Ginny down into the Chamber. By this point, he has absorbed so much of her soul that he can now assume corporeal form, leaving her in a kind of coma. Harry manages to reach the Chamber, where he confronts Riddle and learns the truth. After defeating the basilisk, Harry uses one of its fangs to stab the diary, destroying the Horcrux and restoring Ginny's soul to her body. Later on, Harry realizes that it was Lucius who gave the diary to Ginny in the first place and uses that to trick him into freeing Dobby.
After this, the diary is not seen again until Harry Potter and the Half-Blood Prince. When Harry is learning about Horcruxes, Albus Dumbledore reveals that he still possesses Riddle's diary in its destroyed inert form.
