This book is where the series starts to have a darker twist. Harry suffers from post-traumatic stress disorder, which causes frightening flashbacks and severe mood swings, including a hot temper. Sirius Black, Harry’s godfather, meanwhile has acute depression which is caused by the huge mental toll taken by twelve years in prison and being forced to live in a house where he was abused while growing up. While back at Hogwarts, a chauvinistic dictator who thrives on absolute power takes complete control over the school. She goes to the extremes in discipline, including torturing Harry to get the truth and attempting to suck his soul out. Harry learns that even the most wise, heroic person has hidden faults. Harry also accidentally and indirectly causes the death of his godfather in the attempt to save Sirius’ life. Unrelenting guilt and PTSD are effects from the loss of the only father-figure in Harry’s life.
This is definitely where the series takes a turn for the darker side. Harry is in a dark place in his life and finds himself in literal dark places. Good versus evil is not as clear of a line anymore and Harry must learn how to find his place in all of it.