Harry Potter and the Prisoner of Azkaban (book 3)
For twelve long years, the dread fortress of Azkaban held an infamous prisoner named Sirius Black. Convicted of killing thirteen people with a single curse, he was said to be the heir apparent to the Dark Lord, Voldemort.
Now he has escaped, leaving only two clues as to where he might be headed: Harry Potter's defeat of You-Know-Who was Black's downfall as well. And the Azkban guards heard Black muttering in his sleep, "He's at Hogwarts...he's at Hogwarts.