For twelve long years, the dread fortress of Azkaban held an infamous prisoner named Sirius Black. Convicted of killing 13 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 Azkaban guards heard Black muttering in his sleep, "He's at Hogwarts… he's at Hogwarts." Harry Potter isn't safe, not even within the walls of his magical school, surrounded by his friends. Because on top of it all, there may well be a traitor in their midst.
Harry Potter and the Prisoner of Azkaban, Book 3