28 Magical Facts about Dumbledore You Probably Haven’t Heard Before
Ariana died due to a duel between Albus and Aberforth Dumbledore, and Grindelwald
Albus and Grindelwald were busy making plans for Muggle domination “for the greater good” even while Albus was responsible for taking care of his little sister. Aberforth argued that it was wrong, which obviously infuriated Grindelwald, and lead to a duel between the three wizards. Ariana tried to stop them, jumping between the wizards mid-duel and was struck by a curse and killed. No one really knew whose spell it was that killed her, but Albus never stopped blaming himself for her death.