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.