"Thank you, Newt." Dumbledore handed back what remained in the vial and was the first to step through the arch. The others followed.
The passage opened at the far end into something vast.
A black lake, perfectly still, stretching further than the light could reach. The surface had the quality of dark glass, no reflection, no movement, absolutely nothing to suggest what lay beneath it.
Sirius raised his wand to cast again and the light stuttered, dimming and brightening irregularly. He tried once more and gave up.
"What's happening?"
"The dark magic here is heavy enough to interfere with ordinary casting." Nicolas moved alongside him. "It suppresses. Same principle as before, just applied differently."
When Sirius's wand went dark entirely, the only light source left was the centre of the lake, where a faint green glow emanated from the island in the middle. Everyone understood what it meant. That was where Voldemort had hidden the Horcrux. Where Regulus had replaced it with a fake and paid the price for doing so.
"He even prepared transport for us."
Dumbledore looked down at the shoreline, where a small boat sat waiting. "One person at a time would be the safe limit. Perhaps two at the most."
"Do you still need to go out there?" Tom asked. "We already know it's a fake."
"I want to understand his methods." Dumbledore's tone was thoughtful rather than urgent. "There are techniques Voldemort developed after leaving school that I have no knowledge of. This is a rare opportunity to study them directly."
"Study it after we finish collecting. Can we do that?"
Dumbledore considered for a moment, then nodded.
"Right then." Tom turned back toward the group. "Let's get to work. Nicolas, Newt."
Both men moved forward. Newt turned the dial on his case and opened it. Nicolas stepped closer to the waterline.
Tom transfigured several stones from the ground into birds and sent them skimming out across the surface of the lake. An invisible pressure flattened them before they got far, and the still water came alive immediately. Pale hands shot up from the depths by the dozen, dragging the stone birds under.
Sirius made a sound that was not quite a word.
He had prepared himself mentally for Inferi. He had not prepared himself for this many. The surface heaved, and beneath the swell, heads were rising, white and expressionless, too many to count quickly. A rough estimate from just the near shore came to over a hundred. The rest of the lake did not bear thinking about.
Each one of those had been a person.
Lupin and Sirius stood very still. They had both lived through dangerous times, neither of them easily frightened, but this was different. This was a number. A scale. The name Voldemort, which had always carried weight, settled into something heavier.
"Nicolas!"
"Coming."
Nicolas was already moving, pulling a length of gold rope from somewhere inside his robes with the unhurried confidence of a man who had done considerably stranger things in six centuries. He swung it toward the nearest Inferius, and though his aim left something to be desired, the rope corrected itself in flight, looped cleanly around the pale arm, and then divided into dozens of finer threads that plunged into the water and wound around everything they found.
Tom produced Void rend from his belt.
He took the rope from Nicolas, set his feet, and pulled. The motion was somewhere between a tug of war and someone digging up root vegetables. A chain of Inferi came up from the water in a bundle, wrapped tight, trailing lake water, and he hauled them steadily to shore.
Once they were on the bank he went down the line, one by one, and knocked each one unconscious with a precise tap. Newt was waiting. Each Inferius went directly into the case.
"Next batch!"
Nicolas was already swinging again.
Dumbledore, Sirius, and Lupin watched.
The operation had the rhythm of a factory line. Nicolas caught, Tom pulled and processed, Newt stored. The cave echoed with the sound of water and the regular thud of bodies being collected.
"He genuinely came here to restock," Sirius murmured.
Dumbledore said nothing.
He stood watching the three of them work and tried to determine whether Tom had corrupted Nicolas and Newt, or whether Nicolas, who had never in six centuries been particularly orthodox, had been the one to corrupt everyone else.
He couldn't decide. He laughed anyway.
"What's funny?" Lupin looked over.
"I was imagining Voldemort's reaction," Dumbledore said, "when he discovers that the guardians he spent considerable effort constructing were removed from his cave in this manner."
Sirius grinned. "If I were Voldemort, I'd be coughing up blood."
"Stop laughing and come look at the faces." Tom glanced back at the three of them from where he was working. "One of them is your brother."
The grin dropped from Sirius's face. He crossed the bank quickly and began examining each Inferius as Tom moved them through the process, studying faces, looking for something that matched a memory seventeen years old.
Two hours passed. The lake bottom was more than half emptied and Sirius had found nothing.
He was starting to feel the specific weight of searching for someone in a place like this when an Inferius surfaced near the bank, lean-faced, with long tangled hair loose across its features.
Sirius went still.
Then he was shouting before he had finished thinking.
"That one! That's him! That's Regulus!"