They moved into the Death Eater army the way weather moves — not hurrying, not pausing, simply advancing with the certainty of things that don't need to work very hard.
Dumbledore's wand swept in a long arc. The explosions that followed it were perfectly spaced, each one dropping the next target without touching what was behind it. Grindelwald raised his and struck upward — lightning came down in a staircase pattern, bolt after bolt, each one walking through a row of Death Eaters on the bridge before any of them had time to raise a counter-spell.
The dark creatures were harder to intimidate than the Death Eaters. They charged without the hesitation that strategic intelligence produced. A troll came at Grindelwald's back.
Dumbledore didn't look. He tossed a spell over his left shoulder without breaking the flow of what his right hand was doing, and the troll sat down very heavily.
The two of them had been separated by decades and a great deal of history, and none of that changed the fact that they had once trained together, had once pushed each other to the limits of what either of them could do, and that their spellwork still fit together with the inevitability of long practice.
Grindelwald moved like a conductor — wrist and elbow, wrist and elbow, blue flames arcing from the tip of the Elder Wand in continuous fluid lines that penned enemies in and drove them back. Dumbledore's magic met it and complemented it: moonlight shockwaves where Grindelwald left openings, stone pillars rising to block retreating groups and funnel them into the flames.
He pulled boulders from the hillside — a dozen of them, enormous, hanging in the air. Grindelwald read the intention without being told and wrapped each one in blue fire. Dumbledore launched them.
The impacts sent shockwaves through the enemy lines. The fire from each rock spread outward in a ring and caught the edges of the next group. Within minutes the killing ground in front of the main gates had become a landscape of intersecting flames that only the most stupid or most loyal Death Eater would walk into.
From the battlements, the Hogwarts students watched.
Ron stood with his mouth slightly open. Neville was gripping the stone so hard his knuckles had gone white. Even Draco, who had grown up in a household that spoke of Grindelwald's power with genuine reverence, looked as though he was reassessing several assumptions.
Harry stood apart from them.
He had spotted Hermione emerging from the castle below, running across the grounds, and he'd watched her face change when she saw the bracelet on her wrist whole again, and the equation had rearranged itself in his head.
Kevin was alive.
Which meant the plan was still running.
Which meant Harry had a decision to make, and he was going to have to make it soon.
On the battlefield, Dumbledore cast and spoke at the same time, because forty years of practice had made one independent of the other.
"Your people?" he said.
"Working," Grindelwald said. "Voldemort hid the last Horcrux well — fused it directly into Bellatrix. Extracting it will take time."
"The original plan," Dumbledore said, redirecting a bolt of black fire back into the crowd it had come from, "involved fewer deaths."
"The original plan involved Kevin cooperating the first time I asked," Grindelwald said pleasantly. "He refused. I had to be persuasive."
They walked through a gap in the formation and closed it behind them.
"Voldemort," Dumbledore said. "Kill or capture?"
"Kevin has the sword. He'll manage."
Dumbledore said nothing for a moment, which was its own kind of answer.
He was thinking about Harry. He had been thinking about Harry since Grindelwald first laid out the plan in the headmaster's office — the part he hadn't told Kevin, the part he'd been carrying alone, the quiet impossible arithmetic of what saving Harry might require.
He reached out with a thread of intent, searching through the castle's magic for a specific mind.
Snape received it in a quiet corridor, thirty feet from the main staircase.
"Severus. Take Harry to the Pensieve. Tell him everything."
A pause.
"Don't stand in his way after."
Snape's response came back taut with something Dumbledore recognised as fury held at very close range.
"You're sending him into the fire."
Dumbledore cast a moonlight shockwave that took out a cluster of six Death Eaters simultaneously, bought himself a clear moment, and replied.
"I know. But I believe what's on the other side of the fire may not be death at all. It may be something else."
Silence.
Then Snape was moving.