Which magical object destroys Tom Riddle’s diary in the Harry Potter books?

A basilisk fang destroys Tom Riddle’s diary in the Harry Potter books.

In Harry Potter and the Chamber of Secrets, Harry uses a fang from the dead basilisk to pierce Tom Riddle’s diary. The venom destroys the diary because the object is a Horcrux, a container holding a fragment of a dark wizard’s soul. Destroying the diary also ends the diary’s control over Ginny Weasley and closes the Chamber’s immediate threat.

The basilisk is killed by Harry with the Sword of Gryffindor. Harry then pulls a fang from its mouth and uses it against the diary. This sequence is important because it reveals that certain forms of magical destruction can damage Horcruxes beyond repair.

The diary is the first Horcrux destroyed in the story, although Harry does not yet understand what it is. Readers sometimes confuse the diary with Helga Hufflepuff’s cup, another Horcrux, which Hermione destroys later using a basilisk fang in the Chamber of Secrets.