Item from harry-potter-chamber-of-secrets by
A small black diary containing a fragment of sixteen-year-old Tom Riddle's soul -- it writes back when you write in it, shows you memories, gains your trust, and slowly devours your life to make itself real.
The diary is the most dangerous object in this story. Lucius Malfoy slipped it into Ginny Weasley's cauldron at Flourish and Blotts, not knowing it was a Horcrux -- he thought it was simply a dark artifact that would cause chaos and discredit Arthur Weasley. But the diary is alive, or something like it. It contains a working copy of sixteen-year-old Tom Riddle's personality, charm, and intelligence. Ginny wrote in it because she was lonely -- the youngest child, starting school, crushing on Harry, with no friends to talk to. The diary listened. It sympathized. It asked her about her feelings and made her feel understood. Then it started taking over her body, using her to open the Chamber of Secrets, set the Basilisk on Muggle-born students, kill roosters, and paint threatening messages on walls. Ginny lost time, woke in strange places, and grew weaker as the diary-shade of Riddle grew stronger. Harry destroys it by driving a Basilisk fang through its pages. He doesn't know he's destroyed a Horcrux. He doesn't know there are six more.
A thin, black, shabby-looking diary with a faded date of fifty years ago on the cover. T.M. Riddle embossed in faded gold on the back. The pages are blank until someone writes in it, at which point the diary writes back in neat, slanting handwriting. Ink sinks into the pages and vanishes. When destroyed, black ink pours from the wound like blood.
Also known as: Tom Riddle's diary, Riddle's diary, the diary