Chamber of Secrets
Hogwarts was founded over a thousand years ago by Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw and Salazar Slytherin. For many years, the founders worked together, but then differences began to arise: Slytherin only wanted to admit students coming from purely magical families (Pure-bloods) and keep Muggle-borns out. He was unable to convince the other three founders of this since they thought otherwise and as a result, Slytherin left the school, but not before building a secret chamber in the castle which would be called "The Chamber of Secrets" and inside it concealed a Basilisk.
Later, Slytherin's true heir Tom Riddle was born and he sought to finish Slytherin's noble work of purging the school of all Muggle-borns.
During the school year of 1942-1943, Riddle opened the Chamber and the Basilisk petrified several students and killed a girl, Myrtle Warren. Armando Dippet, who was Headmaster at the time was honking about closing Hogwarts, but in order to keep the school open, Riddle stopped the attacks and framed Rubeus Hagrid and his pet spider Aragog and the former was expelled. The only person who believed that Hagrid was innocent was Albus Dumbledore who kept an annoyingly close watch on Riddle after the expulsion. Flich tried to clean the message away with Mrs Skower's All-Purpose Magical Mess Remover, but to no use.
The first message written by Ginny Weasley said "The Chamber of Secrets has been opened. Enemies of the heir... beware". The message was discovered on 31 October, 1992 on the wall by Harry Potter, Ron Weasley and Hermione Granger when they left Nearly-Headless Nick's deathday party when Harry suddenly heard a voice speaking which Ron and Hermione were unable to hear. They arrived at the corridor where Harry thought he could hear the voice and saw the first message written on the wall, as well as Mrs Norris, the pet cat of the caretaker Argus Filch.
The second message written by Ginny said "Her skeleton will lie in the Chamber forever". When this message was discovered, Minerva McGonagall decided that Hogwarts would surely have to close. After this discovery, Harry and Ron went down into the Chamber of Secrets to rescue Ginny, taking the Defence Against the Dark Arts teacher Gilderoy Lockhart who had unintentionally revealed his lifestyle of fraudulence to the two boys. Harry was parted from the both of them when Lockhart attempted to wipe their memories and then his spell backfired on him and then rock fell and Harry was left on the side of the road to the Chamber while Ron and Lockhart were on the other side.
Harry found Ginny unconscious and was dying, then the memory of Tom Riddle appeared who Harry argued with, then Riddle summoned the Basilisk and tried to attack Harry, but Harry was able to defeat the Basilisk with the help of Albus Dumbeldore's pet Phoenix, Fawkes along with the Sorting Hat and Godric Gryffindor's sword and with the Basilisk fang that pierced Harry's arm which Fawkes was able to heal, Harry stabbed the diary with the fang, the memory of Riddle disappeared, the diary was destroyed and Ginny woke up and was saved. Later these messages was erased from the walls.
It is unknown what happened to the 2 messages after Hogwarts was saved and the Basilisk defeated, but given the fact that Filch tried to scrub the first message off on Hallowe'en night, but couldn't do so, it could be possible that they are still there or else removed in another way.
Behind the scenes
- In the book and the GBC version of the game, only the first message was seen while the second one was only mentioned, but it's possible Harry and Ron saw it when they were heading to Moaning Myrtle's bathroom to find the entrance to the Chamber and to ask her about it too. But unlike in the book and the GBC version, the second message is seen on-screen in the film.
- In the book the message was written with a green letters. In the movie it was written with letters in blood.