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For instance, in The "Half-Blood Prince", Draco spends almost his entire school year trying to fix the Vanishing Cabinet so he will be able to bring back-up Death Eaters into Hogwarts. But couldn't he have used a house-elf to bring them all to Hogwarts by Side-Along Apparition and skip all that fuss and stress and panic attacks? He could have literally had them there whenever he wanted so long as he had a house-elf that would obey him, right? Or any person that had a house-elf that knew where Hogwarts was could get there, right?