He was using Fiendfyre to kill Harry by it had gone too far (basilisk venom and Fiendfyre are the only things that destroy Horcruxes) and destroyed the diadem and it killed himself.
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There is only seven of them: Diary-Harry Ring-Dumbledore Locket-Ron Cup-Hermione Diadem-Crabbe Harry-Voldermort Nagini-Neville
Harry Potter and the Deathly Hallows part 2
Their full names are Vincent Crabbe and Gregory Goyle.
The diadem was hidden in the Room of Requirement
Joshua Herman (Goyle), Jamie Waylett (Crabbe)