The Diary of Tom Riddle

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 (interview in Sydney Morning Herald, 10/2001)

One of the more interesting and dangerous Dark Magic items is the diary of Tom Riddle. This simple diary, originally purchased by Riddle in a Muggle shop on Vauxhall Road in London, was actually a Horcrux. Tom Riddle, who would soon thereafter become Lord Voldemort, placed one torn piece of his soul into the diary, hoping to preserve the work he'd done for years to find and open the Chamber of Secrets.

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.
     -- J.K. Rowling (interview 2/2003)

The diary appeared blank, but when someone wrote onto its blank pages, Tom Riddle as he was during his years at Hogwarts wrote messages back. Lucius Malfoy had kept the diary for years until he had a chance to plant it on Ginny Weasley, slipping it into her Transfiguration textbook. Tom used the diary to influence Ginny into reopening the Chamber and loosing the Basilisk. Harry Potter defeated this memory version of Tom Riddle by plunging the broken, poisoned fang of the Basilisk into the diary. When he did this, all the ink spewed out of it and with it poured out the Dark Magical enchantment (CS17). One of Voldemort's Horcruxes was destroyed.

The stained and torn remnants of the diary were returned to Lucius Malfoy by Harry Potter (CS18).