He had been thinking about Halloween since mid-September.
Not anxiously. The way he thought about projects — identifying the variables, working out the constraints, finding the solution that was both effective and genuinely memorable. The constraint was specific: no casualties, no lasting effects, nothing that produced bad feeling in the morning. The requirement was equally specific: something seen. Something the whole castle experienced at once, that no one could miss, that would become the thing people talked about when they talked about this years Halloween at Hogwarts.
Something visual.
He sat with it in the Room over three evenings in early October, working the problem the way he worked all problems — starting from the effect and working backward to the mechanism. The effect he wanted was: the castle's ghosts, on Halloween night, doing something they had never done before. Not frightening. Not harmful. Something beautiful, and strange, and specific to the night.
The ghosts were not going to cooperate directly. He had a realistic assessment of what the Hogwarts ghosts would and would not do in response to a request from a thirteen-year-old third-year, and the answer was: Nearly Headless Nick might listen, the Bloody Baron would not, Moaning Myrtle was a variable too unpredictable to build a plan around, and the Grey Lady was the only one with the specific combination of intelligence and aesthetic sensibility that might make her an interesting collaborator — but he was not going to ask the Grey Lady for favours when he didn't know what she would want in return.
He wasn't going to use the actual ghosts.
He was going to make new ones.
The mechanism took two more evenings to work out. It required three components: a runic grid laid in advance, a Charm sequence applied during the feast, and a Potion element in the candles — not the regular candles, a specific set he would place himself in the two hours before the feast when the Hall was being prepared and the house elves were working and one additional person moving through the space was unremarkable.
The runic grid would establish the projection field — the area of effect. The Charm sequence, cast from a distance at the activation point, would trigger the sequence. The Potion in the candles would provide the visual medium: when the candles lit and the Charm activated, the smoke from the Potion candles, the specific chemical composition he had been working on for ten days during breaks, would interact with the Charm to produce the forms.
Not white sheets. Not the bluish translucence of actual ghosts. He had thought carefully about what a conjured ghost ought to look like on Halloween, and the answer he had arrived at was: something between a memory and a watercolor. Soft-edged. Luminous. The kind of image that looked like it had always been there and had simply become visible.
He designed thirty of them.
Each one a figure from the castle's history — not invented, not random. He had spent four evenings in the library with Hogwarts: A History and the supplementary architectural texts Bill had recommended and a specific back section of the library that the third-year curriculum didn't require anyone to look at and that contained, in four volumes of very small print, a partial record of the people who had built, taught in, defended, and died in the castle over the past thousand years. He chose thirty. He sketched them — he was not a natural artist, but he had the engineer's capacity for accurate representation, and the sketches were functional enough to load into the runic projection matrices as source images. Founders-era students. A knight whose name appeared only in a footnote about the east wall's original construction. A gardener who had, apparently, been responsible for the first iteration of the greenhouse. A medieval Runes professor, because Babbling would want to know and he thought he might tell her eventually.
He spent a week on the runic matrices. This was the hardest technical work he had done since the summer — the projection grid required twenty-seven interconnected runes, each one calibrated to the specific figure it was projecting, and the connections between them had to be structured so that the activation of the Charm triggered all thirty simultaneously rather than in sequence. He had gotten the theory right on the fifth attempt and tested it with a small-scale model in the Room — three figures, a grid the size of a notebook — on a Wednesday evening, and the three small translucent forms that had appeared above the miniature grid for approximately four minutes had looked exactly like what he intended them to look like.
He had sat looking at them for the full four minutes.
They were very good.
He went to the kitchen on the third Saturday of October.
The kitchen received him with the wide-eyed, attentive focus of a large number of beings who found a student appearing at eight in the evening on a Saturday to be an interesting development. The older elf who had the quality of someone in charge stepped forward.
"How can we help?" she said.
He explained what he needed: help placing the Potion candles in the Great Hall in the two hours before the Halloween feast, in the specific positions he had marked on a diagram. The candles would look identical to the regular feast candles. The placement needed to be exact — the positions were calibrated to the runic grid, and a candle two feet from where it should be would distort the projection on that side of the Hall.
He laid the diagram on the table. The older elf looked at it with the focused attention of someone who was taking a professional reading of a task.
"The positions are marked in blue," he said. "One candle at each mark, mixed in with the regular candles so they're not visible as additions. The runic grid is already in place — I laid it last week in the floor itself, the stone is thick enough that it won't be noticed." He had spent three hours on a Tuesday night in the Great Hall after eleven, which was a story for another time. "The Charm I cast from outside the Hall during the feast. The candles just need to be in the right positions."
The older elf looked at him. "These candles," she said. "What do they do?"
"They produce light and smoke like regular candles," he said. "The smoke interacts with the Charm I cast and produces the visual effect. Nothing harmful, nothing lasting. The effect ends when the candles burn down, approximately four minutes." He paused. "I want it to be beautiful. For the whole castle. That's the entire objective."
Another long look. The wordless consultation with the room.
"Pitts will place them," the older elf said. "The positions will be exact."
"Thank you," he said.
"You will come back," the older elf said, which was not a question.
"Yes," he said.
She looked at him with the quality of someone who had made a decision and was content with it. "Come on a Saturday," she said. "In December. Before breakfast."
He walked out of the kitchen with the specific warmth of an arrangement that had not yet begun but was already good.
The Halloween feast was in its third course when he excused himself.
He had timed it carefully. The feast ran long on Halloween — the abundance of it, the specific atmosphere of a castle that had been doing this for a thousand years and understood the occasion. He had approximately six minutes between leaving the table and the candles being lit for the second wave of the feast, which was when the Hall was at its fullest and the attention of the room would be on the tables rather than the ceiling or the walls.
He walked to the side corridor off the entrance hall — close enough to the Great Hall that the Charm would reach, far enough that he would not be in anyone's line of sight when he cast it. He took out his wand. He looked at the diagram one more time, verifying the sequence in his eidetic memory.
Thirty figures. One activation. Four minutes.
He cast it.
The Charm was not loud. It was not dramatic. It was a precise, structured sequence that he had drilled in the Room for a week and that he delivered now with the focused calm of someone who had done this enough times that the performance was available without effort. He felt the grid activate through the runic structure — the specific feedback of connected runes engaging in sequence — and then he put his wand away and walked back to the Hall.
He was in his seat before it happened.
When it happened, it happened all at once.
The thirty figures rose from the floor of the Great Hall in the slow, certain way of things that had always been there and were simply now visible — luminous, watercolor-soft, each one distinct. The founder-era student near the Ravenclaw table was frozen mid-gesture, one hand raised, looking toward the high table with the expression of someone making an argument. The knight near the east wall was looking at his own hands with the expression of someone who had not expected to find himself here. The medieval Runes professor — he watched Babbling see her and the expression on Babbling's face moved through surprise and recognition with the quality of someone receiving unexpected information.
The castle's actual ghosts, he noted with satisfaction, had gone very still. The Bloody Baron was looking at the figure nearest him — the gardener from the footnote — with an expression that was either recognition or something adjacent to it, and he was not moving, and he was not interrupting, and Ron found this deeply interesting and filed it for later.
The Hall was completely silent for approximately five seconds.
Then it was not silent at all.
The four minutes were the longest four minutes he had spent in the castle. The figures moved — not dramatically, not gesturing or speaking, but with the unmistakable texture of living memory: the shift of weight, the turning of a head, the thing that separated a figure that was present from one that was merely placed. He had not engineered movement into the projection matrices. He had not known how. He did not understand where the movement came from and made a note, in the notebook he kept in his pocket, to think about this carefully.
When the candles burned down, the figures faded the way they had arrived — slowly, with the quality of something becoming less visible rather than disappearing. The last one to go was the founder-era student, who appeared to turn and look directly at the Gryffindor table in the final seconds before the light resolved into nothing.
Ron did not look away.
The Hall was silent for another moment. Then it produced the specific sound of several hundred people trying to process the same thing at the same time, which was a warm, layered, continuous noise that had the quality of something good having happened.
"What," Harry said, beside him, in the tone of someone who had witnessed something and had not yet found the word for it.
"Halloween," Ron said.
"Did you—" Harry started, and then stopped, and looked at him with the expression of someone who had arrived at an answer and was giving it space.
"Happy Halloween," Ron said again.
Harry looked at the ceiling, which was now simply the ceiling — the enchanted night sky, the candles, the usual atmosphere. He looked at it for a moment with the quality of someone experiencing something real and letting it be real. Then he looked at Ron.
"Happy Halloween," he said.