Ravenclaw's lost diadem was the last Horcrux Voldemort intentionally created before the killing curse rebounded, and was destroyed accidentally with fiendfyre that was produced by Vincent Crabbe ...

Voldemort's obsession with Hogwarts and the Hogwarts founders meant that he was desperate to find a relic from each founder to turn into a Horcrux. And he eventually did, turning Ravenclaw's lost ...