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The first Horcrux destroyed — by Harry in the Chamber of Secrets. Now understood as proof that Voldemort split his soul.
Dumbledore explains that the diary was a Horcrux — the first one Harry unknowingly destroyed. Its behavior (possessing Ginny, projecting a memory of Tom Riddle) confirms Voldemort's soul-splitting.
A simple black diary, now destroyed — stabbed through with a basilisk fang.
Also known as: the diary, Riddle's diary