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
be a traitor in their midst.