Sirius stumbling forward, inches from the veil, Voldemort laughing so loud, saying he might take Sirius' body for his own, Boardman yelling he was already dead-
No. No no no nononononoNO!
Harry's vision went grey.
He'd been thinking about it all along, watching the magic flow into the veil as ghosts floated out. Voldemort was at the entrance, pushing against it, but the man was greedy. He wanted to step through it fully formed, human, as full of magic and hate as he'd ever been.
Magic had been disappearing behind the veil for whatever reason, maybe to fuel Voldemort's body, maybe not, but Harry knew instinctively that it was the magic that had to be stopped, not Voldemort.
Harry began drawing the magic into himself, diverting it from the veil, soaking it up like a sponge, like a battery, like parched earth in a rainstorm. All of the movement in the room shifted, and for a moment in time, everyone and thing leaned toward Harry as his body took in the flow of magic in an endless instant, his body fairly glowing with it.
Voldemort screamed, cut off from his supply, and Boardman held fast, unable to move the final step forward with magic pulling so hard against him. Harry's body overflowed and he shaped the magic around him, spinning it into a mass like the swirl of cotton candy at the fair.
Harry wouldn't lose Sirius again, wouldn't have to feel his heart shred as his only real father fell back into nothingness, wouldn't have to live alone and lonely without him-
He had to close the veil. For good.
All the energy Harry had been amassing began to compress, shaping itself to his will until it was a beam of pure white, which Harry directed into the veil. He was working fast, wandlessly, directing magic and transmuting magic itself into stone, filling the veil with unbreakable rock. Voldemort was pushed back, his voice nothing but a hiss before it was gone, and the ghosts were drawn into the beam, becoming part of the energy that was filling up the hole between worlds that was the veil.
It might have gone on for hours or seconds, Harry didn't know, but when he was done there was the sound of popping in his ears, the brief sight of the veil walled over with stone, curtains finally still at last, and then his eyes rolled up in the back of his head and all was silence.