The shield held for longer than anyone had expected.
It was McGonagall's work, fundamentally — the initial structure of it, the interlocking layers — and the dozen professors and senior Aurors feeding power into it were what kept it standing against the sustained assault. Spells rained down like hailstones. Beyond the battlements, the hills around the lake were thick with black robes. Trolls moved at the army's flanks. Dementors circled at altitude, waiting for any gap.
The younger students were gone — evacuated through the secret passages in the first four minutes of lockdown, before the Death Eaters had fully surrounded the grounds. The upper years had stayed. Most of them were at the walls now, casting counterspells until their arms ached.
Harry stood at the main battlements and watched the light-show of a thousand spells hammering the shield simultaneously, and tried to breathe steadily.
"You look remarkably untroubled," said a voice at his elbow.
Slughorn had come up beside him. The man was pink in the face, wand in hand, and looked like he hadn't decided yet whether to be terrified or electrified.
"I'm absolutely terrified," Harry said.
Slughorn blinked.
Harry held up his right hand. In the pulsing light of the shield it was visible — the faint, uncontrollable tremor in his fingers.
"The trick is to not let it decide things for you," Harry said. He lowered the hand. "Kevin's gone. Hermione's not able to help right now. Dumbledore — Dumbledore went after Kevin." He paused. "That leaves this."
He gestured at the castle, the walls, the people on them.
Slughorn was quiet for a moment. Then: "Your mother had that. That fire in her. I always thought she was extraordinary."
"You and Kevin both say that," Harry said, and grinned despite everything. "I'm starting to think it's less a compliment and more a warning."
Then, more quietly: "Professor. Horcruxes. How many did he make?"
The question fell between them like a stone.
Slughorn's expression shuttered. He'd been waiting for it, Harry thought. He'd known this was where the conversation was always going.
"I know you've been careful about this," Harry said. "I'm not asking to blame you for anything. I just need to know how many there are — so I know what we're working against tonight."
Slughorn looked at the shield. At the army beyond it.
Then he looked at Harry, and his eyes were tired and honest and guilty all at once.
"Seven," he said. "He asked me about seven."
Harry nodded.
"Thank you, Professor."
He walked away before Slughorn could say anything else, because there was nothing else useful to say, and there was work to do, and Harry had made his decision somewhere between the Pensieve and the valley and he wasn't going to unmake it standing on a battlement.
Lily Potter had been in Slughorn's advanced class for two years. She'd left him a gift once — a small fish tank, plain glass, nothing special about it on the outside.
"Add water," she'd said, grinning with the particular brightness of a student who knows they've done something clever and is enjoying the moment before anyone else understands it.
He'd poured in a few inches. A single lily petal had risen with the water, and when it hit a certain depth it had transformed — seamlessly, instantly — into a small, perfectly detailed fish that moved through the tank with genuine personality.
"His name is Francis," she'd said. "He likes being talked to."
He had, it turned out, been very easy to talk to. Slughorn had kept him on his desk for years. He'd survived three office reassignments and one very unfortunate incident involving a Niffler. He'd developed, Slughorn was privately convinced, opinions about Potions theory.
And then one morning Francis had turned back into a petal, quiet as a held breath.
No matter how much water Slughorn added. No matter how he coaxed or pleaded.
The news that came a few hours later made the reason clear.
The fish tank had stayed empty since.
The shield broke in the late afternoon, the crack running across it like ice fracturing underfoot — fast and then all at once. The cheering from outside the wall was brief and ugly.
A giant came first, tree-trunk club raised, bellowing. The bridge rattled under its feet.
"Expecto Patronum!"
Dozens of silver shapes erupted along the battlements as the Dementors dove through the gap. They hit the shields of silver light and recoiled, screaming.
The giant brought the club down again. Stone cracked from the east tower.
Acromantulas swarmed the lower walls. The stone golems on the bridge piers met them — not gracefully, but effectively.
Then the horizon changed.
Blue light. A streak of it, moving fast and low.
It hit the approaching giant square in the chest and the creature had perhaps two seconds to realise what had happened before it was simply not there anymore. Blue fire scattered across the stones and the bridge and the front ranks of the Death Eater army, and everyone — attacker and defender alike — went completely silent for one stunned moment.
The fire landed.
Two figures stepped out of it.
"How long has it been since we fought properly side by side?" Grindelwald said. His tone was conversational. He was brushing ash from his sleeve.
"Too long," Dumbledore said. "Let's not waste the occasion."
They walked forward.