Which Horcrux does Harry Potter destroy in the Chamber of Secrets?

Harry Potter destroys Tom Riddle’s diary in the Chamber of Secrets.

The diary is a Horcrux created by Tom Riddle, the student who later becomes Lord Voldemort. It possesses Ginny Weasley and uses her to reopen the Chamber of Secrets beneath Hogwarts.

Harry destroys the diary with a basilisk fang after fighting the basilisk in the Chamber. The fang’s venom is one of the few substances powerful enough to destroy a Horcrux, and the destruction also ends the diary’s control over Ginny.

The diary is Voldemort’s first known Horcrux to be destroyed. It is distinct from the later Horcruxes, including the ring, locket, cup, diadem, and Nagini.