Hon dödades av en rebounding stavning, men ingen vet vem.
I Aberforths ord (betona min):
“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
I Albus ord (betona min):
“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
Ingen av dem går in i detalj på exakt vad som hände, förmodligen för att de inte riktigt känner sig själva eftersom allt hände så snabbt. Men förmodligen var ingen av de tre egentligen riktade mot Ariana (den enda som kunde ha varit Grindelwald, och han hade nog tillräckligt med händerna på de två bröderna), så den stavning som slog henne av en olycka måste ha varit antingen dåligt riktad eller, kanske mer sannolikt, återhämtade sig från en annan stavning eller ett objekt . Och om du tvivlar på att den senare är möjlig, återkalla följande scen i HP och Fire of Fire (betona min):
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: enligt Pottermore anger ovanstående citat från Albus att mördaren troligtvis hade varit 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.