Harry Potter and the Prisoner of Azkaban

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 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.