"DUMBLEDORE!"
The second roar tore through the night like a thunderclap, and this time the Great Hall snapped back to its senses. Dumbledore surged to his feet.
"All Heads of House, escort your students back to your dormitories. Now. Immediately. Nobody leaves without my express permission."
He gave McGonagall no time to ask a single question. Fawkes dropped onto his shoulder in a cascade of flame, and with a flash of fire, Dumbledore was gone.
Tom seized the moment of collective stunned silence and slipped out through the half-open doors. At the same time, he reached into the learning space and opened a connection to Grindelwald.
"Gellert! What in the world are you doing?!"
"Why have you come to Hogwarts of all places?!"
His voice rang through the learning space like a thunderclap of its own, drawing the other three occupants immediately. He seized Grindelwald's consciousness and dragged it in by force.
"What's happening?" Andros looked between them. "Grindelwald came to find you?"
"Find me? He came to find Dumbledore!"
Grindelwald shrugged with the unbothered ease of a man who had just done something entirely reasonable. "Tom, wasn't it you who said you had nothing in particular for me to do? I decided to see some old sights."
"I also told you to gather the Hallows back together!"
"I am gathering them, in my own way." A faint smile crossed Grindelwald's face. "Power commands obedience. Strength earns worship. Do you know why the current situation has remained as stable as it has?"
"It's precisely because I haven't shown my true strength yet. In the hearts of many people, the question lingers. Has he become a relic? A tottering old man looking to go out with one last blaze of madness? Most of the Hallows hold that doubt as well."
"So I intend to shatter it. I intend to show the world that I am still the same Grindelwald I have always been, not diminished but stronger than before. And to accomplish that, there is only one thing that will actually work. I have to fight Dumbledore."
"You just wanted to see your old friend!" Tom was exasperated enough that it came out as a laugh despite himself.
But he couldn't honestly say the reasoning was wrong. In fact it was entirely correct. If you never flexed, why would anyone bother following you?
"Fine. Fight then. Even if Dumbledore kills you tonight, I'm not stepping in."
He withdrew from the learning space, though he left the shared field of vision open.
Grindelwald smiled, completely unconcerned.
Die?
His soul was held within Tom's keeping, bound to this extraordinary space. The space itself was his Horcrux, in a manner of speaking. If his body was destroyed, his soul would return here, and all that would be needed afterward was a new vessel.
As long as nothing happened to Tom, Grindelwald was functionally unkillable.
...
Dumbledore crossed the long bridge with swift, purposeful strides, his expression grave. In the distance, the sky had taken on a strange silver-blue luminescence, unlike anything natural. With a sharp crack, Tom Apparated into existence beside him.
Dumbledore's brow furrowed immediately. "Mr. Riddle, did I not instruct all students to return to their dormitories?"
"Professor, it's Grindelwald, isn't it. Only he and Voldemort could put that look on your face." Tom spoke without any pretense of innocence. "I want to see him in action. I want to see what it looked like when he swept the world."
He was saying it plainly. I want to watch you two fight.
"This is dangerous."
"I can handle myself well enough. You do what you need to do. I'll stay well back."
He stepped away as he spoke, retreating to the midpoint of the bridge. Once on the bridge, the anti-Apparition enchantments of Hogwarts took effect, which meant no one could be pulled away suddenly. Dumbledore watched him go and allowed himself a small measure of reassurance.
Perhaps it was no bad thing for Tom to witness this. Let him see what power at the very peak truly looked like. Let him carry that into the future, so that pride never blinded him, so he never wandered into ruin through overconfidence.
Consider it a live demonstration class.
...
"DUMBLEDORE!"
The third call shook the sky, and Grindelwald finally appeared, vast and terrible above them. Three monstrous birds made entirely of savage fire came with him, the Stymphalian birds of legend reborn in roaring flame, each one enormous enough to blot out a swathe of sky, turning the black night above Hogwarts into a churning sea of silver-blue fire.
"Dumbledore. It has been a long time. I grew a little impatient waiting for our Christmas gathering, so I thought an early visit was perfectly understandable. Don't you agree?"
Grindelwald stood at the centre of the firestorm, looking down from on high, holding Dumbledore's gaze across the distance with an expression full of amusement and mockery and something else, something Dumbledore couldn't quite identify. A kind of superiority that made no immediate sense.
In truth it wasn't superiority so much as pity. Grindelwald pitied Dumbledore, and beneath that, savoured the knowledge of the staggering gap between what Dumbledore knew and what Grindelwald knew.
The one who is truly controlling all of this is standing right behind you, old friend.
Not that Grindelwald spent much time worrying about Dumbledore's wellbeing. If the old man died tonight, it would simply mean he'd turn up in the learning space one of these days. Not the worst outcome.
"Grindelwald. You've never been able to sit still. Is that flight magic? Your own research, or someone else's?"
Dumbledore was genuinely unsettled by that strange look in Grindelwald's eyes, but he would not be caught on the back foot. He pressed forward immediately.
"I have always maintained," Grindelwald said, his voice carrying the unhurried arrogance of a man who had made peace with every choice he'd ever made, "that I never believed I was wrong. I simply lost."
His tone climbed, edged with something wild, something that had been caged for decades and was only now being permitted to breathe again. "I have come tonight to find out whether the years have left you standing still, Dumbledore, or whether you have actually grown!"
The wind came alive around him. His silver-white hair and beard whipped and tangled in the gale, and in his eyes there was a cold and gleaming fire. The Elder Wand appeared in his hand, seemingly from nowhere.
"Then let this be the end of it!" Dumbledore's voice rang out like the toll of a bell. "Your existence is the single greatest source of instability in this world, Grindelwald!"
"HAHAHAHAHA! The end?" Grindelwald's laughter tore through the night. "We are both alone now, you and I. I hear Scamander has come back to England. Tell me, Dumbledore, who is left in the world who still loves you!"
Words gave way to magic.
Dumbledore's wand cracked like a whip, and a spell of tremendous force tore upward through the dark air toward the blazing sky above.