Jon stepped into view almost at once.
"So that's how it is, Headmaster?" Argus Filch blinked, nodded as if he could scarcely credit it, and let go of Harry Potter's collar.
"Yes. My apologies, Argus," Jon said with a small smile. "I forgot to let you know in advance."
"No, no… not your fault at all, Headmaster!" Filch sounded flustered. "I'll be off, then. I won't disturb you!"
He stooped to scoop Mrs Norris into his arms and hurried up the stairs. He did not forget to shoot Harry a vicious glare on his way out.
Filch was gone a moment later.
"I heard Mr Filch bellowing downstairs," Jon said as he came down, his tone mild. "I imagined you and he had fallen out, so I came."
"Thank you… Headmaster…" Harry said, awkward under the rescue.
"Use my name, Harry," Jon replied, amused. "We were classmates once, after all."
"All right," Harry said, a faint grin tugging at his mouth.
"Come on," Jon said, tilting his head toward the corridor.
"Jon," Harry blurted as they walked, "you asked me to meet you at a disused girls' bathroom on the second floor… are you planning to…"
"I'm not planning to be a voyeur, Harry," Jon said dryly, chuckling first at his own joke. "And it's been out of use since fifty years ago, hasn't it? Since I asked you, you ought to know where I'm taking you."
"The Chamber—are we really going to Slytherin's Chamber?" Harry couldn't keep his voice down.
"Lower your voice a touch, Harry. No need to panic the castle," Jon said evenly. "Yes. Professor Dumbledore told me about the Chamber, and I know about your connection to You-Know-Who. That's why I need you."
"Just the two of us?" Harry glanced around. The corridor was empty.
"I'd planned on a few more, but you didn't bring your friends," Jon said with a faint shrug. "We're here."
The door to the bathroom was nondescript, swallowed by shadow and years of neglect. A heap of broken glass bottles and chair legs had piled against the threshold, and a large board reading OUT OF ORDER leaned crookedly across the entrance.
Jon raised his wand. The debris scraped aside one piece at a time.
"Want a hand?" Harry asked, already drawing his wand.
"Sorry, Harry, but we're still in the corridor," Jon said, eyes twinkling. "By school rules, students don't cast spells in the halls. Let's go in."
He took the brass sphere of the handle, turned it, ignored the OUT OF ORDER sign, and pushed the door.
Stale damp closed around them. A long, grimy mirror fractured by spidering cracks faced the door; beyond it ran a row of stone basins with their surfaces flaking. The flagstones were slick. A few candle stubs guttered low in their brackets, throwing a meagre, shivering light that made the wet floor look colder still. The stall doors were scuffed and gouged, paint peeling in strips. One hung crooked, half off its hinge.
"Who is it?" a shrill voice piped from the furthest cubicle. "Who dares trespass on my domain…"
A ghost burst up out of the toilet and hovered. The tone softened at once.
"Oh. Harry. Long time no see. I thought you wouldn't come back before you graduated…"
Harry lingered, mortified and unsure what to say.
"My apologies, Miss Warren," Jon said, lifting his gaze to the ghost. "Myrtle Elizabeth Warren. Mr Potter and I have something urgent to do here. Would you mind giving us a moment?"
"Oh, Headmaster…" Moaning Myrtle looked startled. She retreated and vanished, quick as a blink, back down the U-bend.
"Seems your luck with the ladies isn't half bad today, Harry," Jon said, light as he could manage. "Let's begin. You remember where the entrance is?"
Harry crossed to the basin he remembered and found, just to the side of a copper tap, the tiny engraved serpent.
"I might start speaking something… odd," he warned.
"Parseltongue, then," Jon said with a nod. "Don't mind me."
Harry faced the little snake and hissed. The sounds were thin and sibilant, sliding along the tiles.
Jon didn't idle. He flicked his wand, and a wash of gold light flared behind him. The basilisk had been dead for years, but caution still counted. Better to have Fawkes at his shoulder.
The serpent-etched tap flashed white and began to spin. The entire sink shuddered and sank away. Stone slid back to reveal a chute, a pipe so wide a person could easily drop through.
"Slytherin's Chamber is down there. I'm going first, Jon!" Harry said, and before Jon could answer, he had ducked and gone.
Jon paused, then lifted a hand to the phoenix. He caught one of Fawkes's talons, and together they drifted into the pipe.
Darkness swallowed them. Side channels split away into black in every direction, but none as broad as the central fall. The main pipe twisted and curved, steep and relentless, spiralling down.
After a few minutes the gradient eased. The chute levelled, and they slid out at the mouth of a stone tunnel as black as a well.
"You there, Jon?" Harry's voice echoed down the rock.
Jon didn't answer. He raised his wand, and a clear white light bloomed at the tip. He lifted a finger and gave Harry a small shake of the head.
From ahead, the tunnel breathed out a rank, cloying stench that turned the stomach.