Chosen answer:This is actually never explained in the movie. In the book, Voldemort located Grindelwald in Nurmengard prison so he could learn the whereabouts of the Elder Wand. Grindelwald lied to Voldemort about ever having possessed the Elder Wand, perhaps in a belated effort to protect Dumbledore, his former friend and foe. Voldemort read Grindelwald's mind using Legillimancy and learned that Dumbledore had captured the wand from Grindelwald after defeating him in their epic duel many years before. Voldemort murdered Grindelwald, then deduced that the Elder Wand must have been entombed with Dumbledore.
Chosen answer: This is actually never explained in the movie. In the book, Voldemort located Grindelwald in Nurmengard prison so he could learn the whereabouts of the Elder Wand. Grindelwald lied to Voldemort about ever having possessed the Elder Wand, perhaps in a belated effort to protect Dumbledore, his former friend and foe. Voldemort read Grindelwald's mind using Legillimancy and learned that Dumbledore had captured the wand from Grindelwald after defeating him in their epic duel many years before. Voldemort murdered Grindelwald, then deduced that the Elder Wand must have been entombed with Dumbledore.
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