The diary that writes back
Tom Riddle’s diary possesses Ginny and opens the Chamber. Years later it is the pattern for every other Horcrux: a hiding place for a piece of a soul, destroyed only by something that ruins it beyond repair.
Setup
1Harry Potter and the Chamber of Secrets · The diary Ginny finds, then throws away
A blank diary answers when Harry writes in it. It shows him a memory of Tom Riddle catching the person who opened the Chamber fifty years earlier, and it grows stronger the more it is trusted.
2Harry Potter and the Chamber of Secrets · The Chamber
The diary has been draining Ginny. Riddle comes out of the pages as a memory with a will of its own. Harry destroys the book with a basilisk fang, and the ink and the memory both die.
At the time the book only needs the reader to understand that the diary was dangerous. The word for what it was comes later.
Payoff
Harry Potter and the Half-Blood Prince · Dumbledore’s lessons on the diary
Dumbledore tells Harry the diary was a Horcrux. Riddle had already split his soul as a student, and he used the book both as a weapon and as a way to keep a piece of himself at Hogwarts. The fang that killed the memory is the same kind of destruction the later Horcruxes require.
