Now not only the wizarding world, but the Muggle world itself feels Voldemort's ever-growing power, and Hogwarts no longer seems like a safe haven. Harry suspects that some danger lies hidden within the castle itself, but Dumbledore is more focused on preparing him for the final showdown, which, as he knows, is no longer far off. Together they try to unravel the secret of Voldemort's immortality, and for this Dumbledore invites his old friend and fellow professor Horace Slughorn to take up the post of Potions professor.