The morning after Halloween moved through Hogwarts like a slow tide.
By breakfast, the story had already taken on a life of its own. A mountain troll, defeated by a second-year. A severed arm on the bathroom floor. Spells no seventh-year would dare attempt, cast without hesitation in a flooded corridor that smelled of stone and fear.
Louis had heard four different versions of it before he'd even reached the Great Hall.
He paid them no mind.
He ate quietly, responded politely when addressed, and deflected questions with the same calm he brought to everything else. Cho handled the overflow with practiced grace, redirecting the more persistent students with a warm smile and a firm change of subject. Evangeline simply stared at anyone who lingered too long until they found somewhere else to be.
It worked well enough.
What Louis was actually thinking about had nothing to do with trolls.
...
That evening, the four of them gathered in the Room of Requirement.
The space had arranged itself into something between a study and a war room — low armchairs pulled close to the fire, a long table covered in parchment, and shelves lined with the books they'd accumulated over the past months. Charles had brought tea. Cho had brought a second pot, because she knew Charles's tea was never enough.
Louis stood at the table, wand in hand, tracing slow lines across a blank sheet of parchment.
"Let's go through everything," he said. "From the beginning."
They did.
Cho spoke first, methodical as always. She listed every observed irregularity in Quirrell's behavior since the start of term — the fractured lessons, the absences, the way his hands trembled during certain incantations, the moments where his eyes seemed to lose focus mid-sentence, as though something else had briefly taken the wheel.
Evangeline had gone further. She had been tracking variations in Quirrell's magical aura across different times of day, something she'd started doing quietly after Louis first mentioned the dual signature. Her notes were precise, arranged in a small table she unfolded from her pocket.
"It's more stable in the mornings," she said, smoothing the parchment flat on the table. "More controlled. But by evening, particularly after dinner, it shifts. There's interference. Like something pushing against the boundaries of his own magical presence."
"It's getting worse," Louis said.
"Progressively," Evangeline confirmed.
Charles leaned forward over the table, studying her notes. "So whatever's in there — it's more active at night."
"Which fits," Cho said. "He disappears after dinner at least twice a week. I've checked."
Louis nodded slowly. He had already suspected as much. But having it laid out clearly, in order, with evidence — it made the picture sharper. More uncomfortable.
He set his wand down on the table.
"There's something else," he said. "I've been working on an extension of Visus Gravitas."
The others looked up.
"Passive range. No direct eye contact required. No visible cast." He paused, choosing his words carefully. "If it works the way I think it should, I'd be able to sense magical signatures through a closed door. Through walls, at shorter distances."
Charles raised an eyebrow. "You'd be able to track him without him knowing."
"Without anything knowing," Louis said quietly.
A beat of silence.
"It's not finished," he added. "The range is unstable. It collapses beyond about ten meters and the resonance blurs when there's too much ambient magic nearby." He glanced at Evangeline. "The castle doesn't make it easy."
"Hogwarts has more background mana than anywhere I've ever been," Evangeline agreed. "It would take real precision to filter through that."
"I'm aware." Louis picked up his wand again. "I'm calling it Oculus Silens, for now. I'll keep refining it."
Charles turned his mug slowly in his hands. "And in the meantime?"
"We document everything," Louis said. "We stay close to what we know. And we don't move before we're ready."
Nobody argued.
...
Later, when the others had gone to bed, Louis sat alone in his room.
The tower was quiet. Moonlight cut a pale line across the stone floor, and somewhere below the lake sat still and black. He'd transfigured the far wall into a working surface months ago — it was covered now in sigil diagrams, theoretical frameworks, and lines of magical notation that branched outward like a living thing.
He didn't look at any of it.
He sat on the edge of his bed, wand resting across his knees, and closed his eyes.
The castle breathed around him. That was the only way he'd ever found to describe it — a slow, deep, almost geological pulse beneath the stone and enchantment. Most people never noticed it. Louis had noticed it his first night.
He reached inward, the way Appoline had first taught him, and let his mana settle. Not sharpened, not directed. Just present.
From somewhere far below — not through Oculus Silens, which wasn't ready for distances like that, but through the raw sensitivity he'd carried since childhood — he felt it. Faint. Familiar. Wrong.
The corrupted signature.
It wasn't moving. It was simply there — the way a bruise was there, the way rot beneath wood was there. Quiet for now. Patient.
Louis opened his eyes.
He sat with that for a long moment.
Then he reached, not outward but sideways — through the thread he knew better than anything else he'd built. The mental link stirred and opened like a door on a warm evening.
"You're still awake," Fleur said.
Her voice in his mind was soft, slightly surprised. He could feel the faint texture of her own tiredness through the bond — it was late in France too.
"I was thinking," he said.
"You're always thinking."
"Tonight more than usual."
A pause. He felt her attention shift, sharpen. She knew his tones, even the ones that existed only in thought.
"Tell me," she said.
He did.
Not everything — not the details of the meeting, not Evangeline's charts or the specifics of Oculus Silens. But the shape of it. The growing certainty that something beneath the castle was moving toward a conclusion. That whatever was living inside Quirrell was becoming less patient.
Fleur listened without interrupting.
When he finished, the link was quiet for a moment.
"And Dumbledore?" she asked finally.
"He knows," Louis said. "He's known from the start. I'm certain of it."
"Then trust him."
"I do." He paused. "Mostly."
He felt something that might have been a quiet laugh on her end. Not dismissive — warm.
"Promise me something," she said.
"That depends on what it is."
"Louis."
"I'm serious. I don't make promises I haven't considered."
Another pause. Longer this time.
"Promise me," she said, "that when whatever this is reaches its end — you won't try to face it alone."
Louis looked out at the lake through his window.
He thought about the corridor earlier that year, the Slytherin first-years frozen under Vitus Gravita. He thought about Halloween, the troll, the arc of blue light in a flooded bathroom. He thought about the way Voldemort's consciousness had noticed him during that first scan — a cold, ancient awareness turning in his direction like something enormous turning an eye.
"I promise," he said.
He meant it.
"Good," Fleur said. And then, softer: "Bonne nuit, Louis."
"Bonne nuit."
The link quieted. The room settled back into its moonlit silence.
Louis set his wand on the desk. He pulled out his research journal, opened it to a fresh page, and began to write.
At the top, in small, precise letters, he wrote: Oculus Silens — Development Notes, Day 1.
Beneath it, he sketched the first sigil framework.
He worked for another hour before the candle burned low enough to remind him that sleep existed.
He blew it out. Lay down. Looked at the ceiling.
The corrupted signature was still there, faint at the edge of his perception.
Patient, he thought. So am I.
He closed his eyes, and eventually, the castle's slow breathing carried him under.