Harry Potter and the Chamber of Secrets

The second in the Harry Potter series. Harry Potter and his friends Ron Weasley and Hermione Granger discover that someone called the "heir of Slytherin" is unleashing a monster to purge Hogwarts of all "mudbloods", a derogatory term for wizards with Muggle (non-wizard) parents. Most people think Harry is Slytherin's heir because he can talk to snakes. Harry suspects that his enemy fellow student Draco Malfoy might be the heir of Slytherin but eventually discovers that it is the memory of the Dark Lord Voldemort's sixteen year old self who is Slytherin's heir.