Ela foi morta por um feitiço de ressalto, mas ninguém sabe de quem.
Nas palavras de Aberforth (ênfase minha):
“And there was an argument… and I pulled my wand, and he pulled out his, and I had the Cruciatus Curse used on me by my brother’s best friend – and Albus was trying to stop him, and then all three of us were dueling, and the flashing lights and the bangs set her off, she couldn’t stand it – ”
The colour was draining from Aberforth's face as though he had suffered a mortal wound.
“ – and I think she wanted to help, but she didn’t really know what she was doing, and I don’t know which of us did it, it could have been any of us – and she was dead.”
-- HP and the Deathly Hallows, Chapter 28: The Missing Mirror
Nas palavras de Albus (ênfase minha):
“The argument became a fight. Grindelwald lost control. That which I had always sensed in him, though I pretended not to, now sprang into terrible being. And Ariana… after all my mother’s care and caution… lay dead upon the floor.”
[...]
"Oh, not death," said Dumbledore, in answer to Harry's questioning look. "Not what he could do to me magically. I knew that we were evenly matched, perhaps that I was a shade more skilful. It was the truth I feared. You see, I never knew which of us, in that last, horrific fight, had actually cast the curse that killed my sister. You may call me cowardly: You would be right, Harry. I dreaded beyond all things the knowledge that it had been I who brought about her death, not merely through my arrogance and stupidity, but that I actually struck the blow that snuffed out her life.”
-- HP and the Deathly Hallows, Chapter 35: King's Cross
Nenhum deles detalha exatamente o que aconteceu, presumivelmente porque eles não se conhecem como tudo estava acontecendo tão rápido. Mas presumivelmente nenhum dos três estava realmente mirando em Ariana (o único que poderia ter sido Grindelwald, e ele provavelmente tinha o suficiente em suas mãos lidando com os dois irmãos), então o feitiço que a acertou por acidente deve ter sido ou mal direcionado ou, talvez mais provavelmente, recuperado de outra mágica ou objeto . E se você duvida que o último é possível, lembre-se da seguinte cena em HP e no Cálice de Fogo > (ênfase minha):
For a split second, they looked into each other’s eyes, then, at exactly the same time, both acted.
"Furnunculus!" Harry yelled.
"Densaugeo!" screamed Malfoy.
Jets of light shot from both wands, hit each other in mid-air, and ricocheted off at angles – Harry’s hit Goyle in the face, and Malfoy’s hit Hermione. Goyle bellowed and put his hands to his nose, where great ugly boils were springing up – Hermione, whimpering in panic, was clutching her mouth.
EDIT: de acordo com Pottermore , a citação acima de Albus sugere que o assassino era mais provável que tenha sido Grindelwald :
Quite who cast the spell that killed Ariana is unclear, although when Harry meets Albus Dumbledore in King’s Cross after he is nearly killed during the Battle of Hogwarts, the old Headmaster strongly hints that it was Grindelwald who shot the mortal curse.