Harry suffers another emotional blow , when his godfather , Sirius Black is killed during a battle with Death Eaters at the Department of Mysteries , but Harry ultimately defeats Voldemort 's plan to steal an important prophecy and helps uncover Umbridge 's sinister motives . Harry suffers another emotional blow , when his beloved godfather , Sirius , is killed during a duel with Sirius ' cousin , the Death Eater Bellatrix Lestrange , at the Department of Mysteries , but Harry ultimately defeats Voldemort 's plan to steal an important prophecy .