Before receiving McGonagall's invitation to come to the ivy-draped underground dock beneath the castle, Snape's past few days had been rather smooth.
During this period, he had managed to maintain a delicate balance among his "friends." Just moments ago, he had once again coaxed and tricked Tom Riddle's diary into performing another round of treatment on Nagini, the curse upon her was gradually being lifted.
Everything seemed to be proceeding exactly according to his plan.
Or at least, that was what he believed.
During one of their exchanges, Tom had assumed that the enemy Snape had encountered earlier was himself, and even hinted that he knew certain secrets about the Dark Lord. He suggested that Snape abandon his prejudice and take him to join Voldemort.
But when Snape pressed for specific reasons, Tom had merely replied vaguely that his original self had once made contact with the Dark Lord, without offering any convincing explanation.
"I've already told you, Tom," Snape wrote upon the page, "I'm not pureblood. The Dark Lord would never truly accept me."
"Trust me," Tom replied, "the Dark Lord would greatly appreciate your talents. What he values is ability, not blood."
"Perhaps. But I'll need time to think about it." Snape finally wrote, then snapped the diary shut and tucked it into the inner pocket of his robe.
...
On his way back to the castle, a tense voice broke through Snape's thoughts.
"Snape!"
Snape looked up to see McGonagall striding rapidly toward him, her expression unusually grave.
"Professor?" Snape asked in confusion.
"Finally found you." McGonagall didn't even slow her pace, merely signaling with her eyes for him to follow. "Come with me. Professor Dumbledore has given instructions."
Snape raised an eyebrow. He hadn't seen Dumbledore around the castle for several days. What could this sudden directive be?
"Professor, could you tell me´-"
"Not now." McGonagall cut him off, turning down a narrow corridor he had never noticed before.
They passed through several hidden hallways before arriving at a place beneath the castle that Snape had never seen.
A stone wall covered in ivy blocked the way. McGonagall raised her wand and recited a long, intricate incantation. The vines writhed aside like living creatures, revealing a pitch-black door.
A cold, damp wind poured out from within, revealing a flight of stone steps leading down.
"What is this?" Snape asked.
"The Secret Dock of Hogwarts," McGonagall replied in a low voice. "Built during the era of the Four Founders, meant for emergency evacuation."
Descending the stone steps, a wide underground dock came into view. The surface of the Black Lake shimmered faintly like tin foil under the scattered glow of magical lights.
A centuries-old wooden ship was moored at the edge. Professor Flitwick was directing piles of fresh timber to reshape and mend the damaged hull through Transfiguration.
Dozens of house-elves were popping in and out of existence with soft cracks, carrying barrels of drinks and wooden crates of various sizes up the gangplank into the ship.
The entire scene was both busy and orderly, as if they were preparing for a large-scale voyage.
"What's all this for, McGonagall?" Snape asked softly, his heartbeat quickening.
He noticed delicate protection runes carved into the newly repaired sections of the ship. In his memory, there had never been such a scene in the original storyline.
"Four days ago, Professor Dumbledore left the school," McGonagall said, stopping and turning toward him. "Before he left, he told me that if he didn't return within a week, I must organize all Muggle-born students to leave Hogwarts." She adjusted her glasses and added, meeting Snape's eyes directly, "He said you would help me organize the students, and that I was to bring you along."
"Evacuate? Why?" Snape's voice came out dry. "Where is Professor Dumbledore? What has he gone to do?"
Everything before him had deviated completely from what he knew, and from the direction he intended to steer events. Things were moving toward an unpredictable future.
McGonagall pressed her lips together and shook her head.
"I don't know," she said. "He didn't say and I didn't have time to ask." Her gaze was firm. "But you know as well as I do, when Albus Dumbledore decides to handle something personally..."
What on earth had Dumbledore gone to do? Snape fell silent. He had always believed that, armed with his full knowledge of the "story," he could guide events toward the best possible outcome.
He had obtained the means to destroy Horcruxes and had personally destroyed two of them; he even carried another in his robes. He had saved Nagini and changed young Barty's fate...
From here on, he should have been able to move forward without relying on Dumbledore's delicate, failure-prone web of plans.
A rosy future should have been slowly unfolding before him.
But now, that great wizard, the one who sheltered everyone from storm and shadow, had vanished. And McGonagall was preparing to evacuate the students. The future had shifted into the unknown.
"You said 'a week', that means three days remain," Snape said.
"These past few days," McGonagall replied, "even my Patronus hasn't been able to reach him. That has never happened before. We must be ready to depart in three days."
"No." Snape raised his voice slightly. A few house-elves froze, and Professor Flitwick turned around in surprise.
"You don't want to leave?" McGonagall frowned.
"No. I mean, we can't just leave like this," Snape said firmly, realizing he would have to clean up the Headmaster's mess again. "Professor Dumbledore will return."
"I hope so too, Severus." McGonagall's expression softened, it was the first time she had called him by name. She stepped forward and placed a hand on his shoulder. "But we must prepare for the worst. If he hasn't returned in three days, "
"Are we just supposed to wait?" Snape interrupted. "Are we supposed to flee Hogwarts like beaten dogs, pinning all hope on Professor Dumbledore?"
He realized at once that he'd gone too far. McGonagall's grip tightened sharply.
Her eyes widened. Flitwick lowered his wand, staring in shock.
"I'm sorry, Professor," Snape said quickly, regaining composure. "I got carried away."
"It's all right," Flitwick said, stepping forward, his tone surprisingly calm. "We're all under great pressure."
"Severus," McGonagall nodded, taking a deep breath. "I understand how you feel. But as teachers, our foremost duty is to protect the students, and to follow Dumbledore's orders."
"We can't just abandon him," Snape insisted. "We have to find him."
"But we don't know where he is," McGonagall said helplessly. "None of our methods can reach him."
"Have you tried asking them for help?" Snape asked, glancing at the busy house-elves.
"What?"
Snape had already approached the nearest elf, who was staggering under a stack of books taller than himself.
"Hello there, elf. What's your name?" Snape crouched down and helped the creature place the books on the ground.
"Koukou, sir."
"Sorry, " Snape blinked. "Kokou?"
"Koukou, I have a question. Can you sense where Professor Dumbledore, the Headmaster, is right now?"
The elf blinked his bulbous eyes, then closed them, his long ears twitching.
After half a minute, he snapped his eyes open and nodded vigorously. "I can feel him! Koukou can feel the Headmaster, sir!"
McGonagall and Flitwick exchanged startled, delighted glances. "House-elves... how could we have overlooked them..."
"Professor," Snape turned to McGonagall, "did you instruct the elves to keep all this secret?"
"Of course." She nodded. "Every elf involved has been ordered to strict secrecy."
Snape looked back at Koukou. "Koukou, can you take me to Professor Dumbledore?"
The elf hesitated, glancing at McGonagall. She gave a small nod.
"Yes, sir," Koukou said, extending his small hand.
"Take me there." Snape reached out.
Just as their hands were about to touch, McGonagall stepped forward and grasped the elf's other hand.
"Professor?" Snape looked up in surprise.
"I'm coming too," she said, meeting his eyes. "How could I let a student go off alone on such a dangerous errand?"
Snape nodded.
In the next instant, all three vanished from the underground dock.
This time, Apparition felt unusually sluggish and uncomfortable, as though they were passing through a layer of thick syrup.
When Snape's feet touched solid ground again, he instinctively drew his wand, scooped up Koukou, and rolled aside.
McGonagall also moved quickly, wand raised toward the darkness.
No attack came. Only silence, and a faint green glow in the distance.
After a pause, Snape set the elf down gently, patting his head to signal him to hide.
He fired a Stunning Spell toward the green light. As the red beam sliced through the dark, he caught sight of their surroundings, a vast cavern, a black lake at its center, the eerie green light emanating from a small island.
He recognized the place at once. It was the cave where Voldemort had hidden Slytherin's locket Horcrux.
He hadn't expected Dumbledore to have tracked it down already. Could it be that he'd drunk the potion from the stone basin? If so, that was actually good news, even Kreacher had survived the potion's effects.
"Lumos," McGonagall whispered, and light flared from her wandtip.
It illuminated a nearby rock. Dumbledore lay slumped against it, eyes closed, face deathly pale. Beside him, a strange man held a small boy in his arms, both unconscious.
"Albus!" McGonagall cried, rushing toward him.
She knelt at Dumbledore's side and immediately began casting a series of healing spells.
"He's alive," she said with palpable relief. "But very weak... I don't know why he won't wake."
Snape, meanwhile, kept his wand trained on the two strangers, checking swiftly for wands or magical objects.
The man looked to be in his early thirties, wearing an ordinary Muggle shirt and jeans. The boy, no older than ten, wore a cartoon-print T-shirt. Their calloused hands bore no signs of wand use.
"They may be Muggles caught up in this," he said quietly, casting healing spells on both. The boy stirred faintly but did not awaken.
"We must leave at once," McGonagall said, scanning the area with detection charms. "No one else is alive here." She levitated Dumbledore and the two Muggles, calling out, "Koukou, come here!"
But Snape's gaze drifted toward the eerie green glow at the lake's center.
In his memory, that was where the stone basin filled with emerald poison stood, one had to drink it to obtain the locket. If Dumbledore had already come here, then...
"Wait," Snape said, stopping the elf. "Koukou, do you see that island? Can you go there and look? See if there's a stone basin, and what's inside it?"
"What?" McGonagall frowned. "Severus, perhaps we should just leave, "
"But we must know whether Professor Dumbledore achieved his purpose," Snape said, shaking his head. "Please, Koukou."
The elf looked fearfully at the dark lake, but nodded bravely and vanished with a crack.
Moments later, he reappeared at Snape's feet.
"There is a stone basin, sir," the elf reported. "On a black pedestal. There's a little green water inside, glowing. Nothing else."
"Only a thin layer?" Snape pressed.
"Yes, sir. Very little," Koukou confirmed.
A complicated feeling welled within Snape.
If Dumbledore had drunk from the basin, the potion should have replenished itself, unless Voldemort had chosen this cave but had not yet fully activated its enchantments.
In that case, since Dumbledore had already found this place, Voldemort would never again use it to hide the locket. A Horcrux that Snape had thought he could easily secure had now vanished from his grasp.
But if Dumbledore hadn't drunk the potion, why had he fallen unconscious? And who were those two Muggles?
As for Regulus... he would have to fend for himself now.
"Severus?" McGonagall called anxiously.
Snape fired a spell into the lake; apart from ripples, no Inferi rose from the depths.
"Let's go," he told the elf. "Take us directly to the Headmaster's office. Koukou?"
The elf nodded and stretched out his hands.
Snape and McGonagall simultaneously levitated Dumbledore and the Muggles, grasped their hands, and then held onto the elf.
With a soft pop, they vanished from the cavern.
When the familiar surroundings of the Headmaster's office materialized around them, McGonagall finally exhaled deeply.
She carefully Transfigured a wooden bed, layering it with soft feather cushions, and laid Dumbledore upon it.
Then she strode to the window, raising her wand. Three silver cats burst forth from its tip, each with spectacle-shaped markings around their eyes.
The three Patronuses leapt through the window and scattered in different directions, filling the night sky with silver light.
"Madam Pomfrey and the others will be here soon," McGonagall said, her voice weary but relieved. "He'll be fine. Thank you, Severus."
"It's nothing," Snape said, nodding as he settled the Muggles and looked down at Professor Dumbledore. He couldn't suppress a sigh. His thoughts drifted away. The locket's whereabouts were now a mystery, what steps should he take next?