<P> Ron, Hermione, and Harry join the effort to save Hagrid's hippogriff, Buckbeak, from being executed for attacking Draco Malfoy, after Draco provoked him . Their efforts are unsuccessful, but Scabbers reappears shortly after they hear Buckbeak being executed . </P> <P> Ron chases Scabbers, only to be attacked by a big black dog, which Harry has seen several times before . The dog drags Ron through a tunnel under the Whomping Willow into the Shrieking Shack . Harry and Hermione follow, and there is a brief standoff when they find Ron with Sirius Black, who has transformed from the dog . Lupin enters, and they explain the situation to Harry and his friends: Lupin is a werewolf, which led to his friends James Potter, Sirius Black, and Peter Pettigrew becoming animagi . Lupin explains that Scabbers is Pettigrew in his animal form; he has been hiding from Black, whom he had framed for betraying Harry's parents and murdering the thirteen Muggles . </P> <P> Snape arrives to apprehend Black but Harry, Ron, and Hermione knock him unconscious . Lupin and Black transform Pettigrew back into human form and prepare to kill him, but they are stopped by Harry, as he feels his father would not have wanted it . He convinces them to send Pettigrew to Azkaban instead . </P> <P> As they move back toward Hogwarts, Lupin turns into a werewolf and becomes violent, having missed a dose of his Wolfsbane potion . Pettigrew escapes again, and Black prevents Lupin from attacking the others in werewolf form . Some Dementors approach Harry, Ron and Hermione . </P>

What is the climax of harry potter and the prisoner of azkaban