Rowena Ravenclaw's diadem Horcrux was destroyed when Vincent Crabbe unleashed Fiendfyre (cursed fire) in the Room of Requirement during the Battle of Hogwarts. The Fiendfyre incinerated the diadem - the only way to destroy this particular Horcrux. Basilisk venom (via Gryffindor's sword) destroys Horcruxes but was not used on the diadem, and the Killing Curse cannot destroy Horcruxes.