Gregorovitch once had the Elder Wand, he knew—the latest glimpse into Voldemort's mind told him that. Grindelwald stole it, but then he lost the Elder Wand to Dumbledore in the duel. And now the wand was resting with Dumbledore's body back at Hogwarts. How he would open the marble tomb, he didn't know, but he would come up with something.