Not until the moonlight had shifted significantly across the floor did Tom Riddle leave the Forbidden Forest and come to the second-floor girls' bathroom corridor.
He stopped in front of a wall and used his wand to leave glowing red characters on the stone: The Heir of Slytherin shall live forever, and the bones of the impure shall remain in the Chamber of Secrets forever.
Snape watched as Tom Riddle entered the bathroom and opened the Chamber of Secrets using Parseltongue. The washbasins suddenly split apart to reveal a massive opening, and Tom Riddle stepped inside, the entrance sealing shut the moment he was through.
Chapter 140