Whose diary was one of Voldemort's Horcruxes?

The story behind the answer

Tom Riddle is Voldemort’s birth name, and his old diary held a fragment of his soul—making it a Horcrux. In Chamber of Secrets, the diary manipulates Ginny Weasley and channels teenage Riddle’s memory to reopen the Chamber. Harry destroys the Horcrux with a basilisk fang, which damages the soul fragment beyond repair.

Context to remember: “Tom Marvolo Riddle” rearranges to “I am Lord Voldemort,” linking the diary’s owner to the Dark Lord. Mnemonic: “Riddle’s diary = Voldemort’s riddle for immortality.” The diary was one of Voldemort’s earliest Horcruxes and the first to be destroyed, teaching that basilisk venom and Fiendfyre can permanently destroy Horcruxes.