In what Harry Potter book was a Horcrux first encountered?

The story behind the answer

The first Horcrux appears in The Chamber of Secrets: Tom Riddle’s diary. Although the term “Horcrux” isn’t introduced until The Half-Blood Prince, the diary is later confirmed as one—it manipulates Ginny Weasley and manifests a preserved memory of Riddle, showing how a soul fragment can act independently.

Horcruxes are dark objects made by splitting the soul through murder and hiding the fragment to gain immortality. Voldemort created several; the diary was both a test and a weapon, designed to reopen the Chamber. Harry destroys it with a basilisk fang, whose venom irreparably damages Horcruxes. Memory hook: “Diary in the