The prisoner's dilemma

"Robin Sear, I think you know why, don't you?"

Luo Bin casually pulled up a chair and sat opposite him. "I'd like to tell you, but I don't know where to start or how to explain it!"

"I'll ask, and you answer. Why did you, Harry, Hermione, and Ron rush upstairs last night? Don't give me that nonsense about being cold and wanting another layer."

Their discussion on the fifth floor last night had ended with an agreement to keep the "strange voice" secret for now. But knowing the "old bat," he wouldn't teach Luo Bin anything about potions until he got to the bottom of it. Worse, he would fix those hollow, cold, sinister eyes on him, making it impossible for him to concentrate on anything.

Besides, Luo Bin had enrolled at Hogwarts to properly study magic. He didn't want the whole school gripped by an unexplained panic for a long time to come. After wrestling with the decision, he took out the Shark-Fin Headphones.

"Is this... an earplug?"

Snape picked up the little black object and examined it for a while before asking uncertainly.

"Uh, you can call it a headphone or an earplug. It has a remarkable enchantment inside that helps the wearer block out various sounds, like conversation, chewing, insects, rain, and so on. At the same time, it lets the wearer hear very unusual sounds, such as whispers inside stone walls."

Snape stopped turning the headphone over in his hand and followed Luo Bin's gaze to the dingy stone wall. His brow furrowed as he considered what sort of creature could move through stone walls.

"What does any of this have to do with last night?"

"Everything. The dungeon Nick booked was big, but even that couldn't comfortably fit hundreds of ghosts. Ghosts aren't confined