The letters on the wall of the first-floor corridor
The First-Floor Corridor is the main corridor on the first floor of Hogwarts castle.
From the Story[]
"On Hallowe'en, a message of warning to the school is daubed on the wall of a corridor and Mrs Norris is found hanging from a torch bracket, Petrified. Filch tries to remove the foot-high words but is unable to, even when using magical cleaning products."