The book builds up suspense around the mystery of the Chamber, the attacks on Muggle-born students, and the return of a dark force. The reader is led to believe various characters could be the culprit, including Draco Malfoy, Gilderoy Lockhart, or even Harry himself.
However, the truth is far more shocking. When Harry confronts Tom Riddle's diary, he discovers that the diary is a Horcrux containing a piece of Voldemort's soul. Tom Riddle then reveals his plan to purify the wizarding world by eliminating the Muggle-born, a cause he believes Slytherin himself would have supported.
This revelation is an upset because it:
* Introduces the concept of Horcruxes, a crucial element in future books.
* Reveals the extent of Voldemort's evil and his connection to the Chamber of Secrets.
* Forces Harry to confront his own connection to Voldemort and the prophecy that binds them.
The upset also intensifies the conflict, pushing Harry and his friends into a desperate race against time to stop Voldemort from completely taking over Hogwarts and completing his evil plan.