What he did was not a single spell.
It was two, simultaneous, interlocked — the specific technical challenge of maintaining two distinct magical constructs at the same time, each requiring sustained output, each dependent on the other functioning correctly.
The first was the platform. He conjured it at the depth, at the bearing, sized for one small person — a flat solid surface, conjured into the water rather than above it, which required a different approach than standard conjuration because water resisted the occupation of its space by solid objects that had not entered it through its surface. The resistance was real and had to be overcome not through force but through the specific precision of a conjuration that arrived rather than intruded.
He felt it settle at depth. Rough bearing, as promised. He adjusted by two degrees and felt the adjustment land closer to where the tracking charm said she was.
The second was the displacement column. He built it above the platform — a rising pressure, water pushing water in a directed upward column, the physics of it requiring not a Charm but a sustained Transfiguration of the water's own movement pattern. Not forcing the water upward. Convincing it to go upward. The distinction was not philosophical. It was the difference between a thing that held for sixty seconds and a thing that held for sixty seconds and then catastrophically failed, and the one he was doing was the former.
Both constructs running simultaneously. Both sustained. The platform rising through the displacement column, the column pushing up beneath it, the whole system a single integrated movement carrying one small girl from forty feet down to the surface of the Black Lake in February at Hogwarts School of Witchcraft and Wizardry.
The lake surface erupted.
Not violently — not the explosion of something going wrong, but the specific dramatic emergence of something going right. The displacement column broke the surface first, a circular disturbance that pushed outward, and then the platform rose through it — a flat solid disc of water-dark material, streaming, the size of a dining table, and on it a small girl of eight years old, still in the enchanted sleep the task required of its hostages, entirely unharmed, arriving at the surface of the lake with the specific quality of something that had been somewhere and was now here.
The platform reached the surface and stopped. He held both constructs for four seconds — long enough to confirm she was clear of the water, long enough for the staff to register what had happened and begin moving — and then he released them.
The platform dissolved. The displacement column collapsed into ordinary lake water. Gabrielle Delacour floated for a moment on the surface, sustained by the enchantment that the task had placed on her, and the staff were already in the water from the bank's edge, reaching for her, pulling her in.
Ron lowered his wand.
He was aware, in the specific detached way of someone whose body was reporting information he did not particularly want to examine in detail right now, that the two-construct sustained conjuration had cost what the two-construct sustained conjuration always cost in practice, which was considerably more than it had cost in theory. His hands had the specific quality of something that had been working hard and would prefer to stop. The world had the slight quality of something that had been asked to be two places at once and was resolving back to one.
Hermione's arm was there before he had to find it.
She had moved when he raised his wand and was standing at his left side with the specific quality of someone who had decided where she needed to be and had been there for the duration. He did not lean on her dramatically. He simply allowed the contact, which was the honest version of what was happening, and she held it without comment.
'Two constructs,' she said, quietly.
'Yes,' he said.
'Simultaneously.'
'Yes.'
She was quiet for a moment. 'You should sit down.'
'In a minute,' he said.
She did not argue. She kept her arm where it was.
Dumbledore reached him before the medics had finished with Gabrielle.
He had not been moving quickly — Dumbledore never moved quickly, in the way of someone who had learned decades ago that moving quickly communicated urgency and urgency communicated loss of control, and he had no interest in communicating either. But he had moved directly, which was its own form of speed, and he was at Ron's side within ninety seconds of the platform dissolving.
He looked at Gabrielle, being received by the marshals at the bank's edge, conscious now, coughing lake water, alive.
He looked at Ron.
'I considered,' Dumbledore said, with the mild tone he used when he was saying something that had more than one layer to it, 'going in myself.'
'I thought you might,' Ron said.
'You acted first,' Dumbledore said.
'You would have gotten your robes wet,' Ron said. 'February lake. It seemed unnecessary.'
Dumbledore looked at him over his half-moon spectacles with the expression that was not quite the benign mild thing but the older thing underneath it. 'Two sustained simultaneous conjurations at depth,' he said. 'Without preparation time. Without line of sight.'
'I had a bearing,' Ron said. 'It was imprecise.'
'Yes,' Dumbledore said. 'I noticed.' He paused. The lake was doing its ordinary work behind them — cold, dark, entirely unconcerned with what had just happened in it. 'You are fifteen years old, Mr Weasley.'
'Yes,' Ron said.
'I find myself wondering,' Dumbledore said, with the quality of someone who was not wondering at all but was choosing to frame a statement as a wonder, 'what you will be capable of at twenty-five.'
Ron looked at the lake. He thought about the white void and the being and the wheel, and about the specific version of twenty-five that existed in the timeline he had come from, and about what the current version might look like instead.
'Hopefully less necessary,' he said.
Dumbledore was quiet for a moment. Then: 'Yes,' he said, with a weight in the word that was not quite sadness and not quite hope but was the specific thing that lived between them. 'Hopefully.'
He moved away to attend to the rest of the occasion with the grace of someone who had said what he came to say and was satisfied with the saying.
Hermione looked at Ron.
'You should sit down now,' she said.
'Yes,' he said, and did.