For years the Purebloods in the Wizarding world have claimed to be superior because of their Families and Heritage. After the events in the Chamber of Secrets Harry is astonished to find that there is some truth to this theory and that his encounter with the Basilisk in the Chamber of Secrets begins to shake loose some amazing abilities passed down from his ancestors.
What would have happened if Sirius Black hadn't tried to kill Peter Pettigrew? Still sent to Azkaban he appeals to his Grandfather who decides that it's time that the Black Family shaped up to their Noble name. They adopt an infant Harry and raise him to be the Pureblood Heir he is.