I took the underline parts directly from the book, an exact quote...

Only one couple were still battling, apparently unaware of the new arrival. Harry saw Sirius duck Bellatrix's jet of red light: He was laughing at her. "Come on, you can do better than that!" he yelled, his voice echoing around the cavernous room.

The second jet of light hit him squarely on the chest.

The laughter had not quite died from his face, but his eyes widened in shock.

Harry released Neville, though he was unaware of doing so. Harry jumped to the ground, pulling out his wand, as Dumbledore turned to the dais too.

It seemed to take Sirius an age to fall. His body curved in a graceful arc as he sank backward through the ragged veil hanging from the arch….

And Harry saw the look of mingled fear and surprise on his godfather's wasted, once-handsome face as he feel through the ancient doorway and disappeared behind the, which fluttered for a moment as though in a high wind and then fell back into place.

Harry heard Bellatrix Lestrange's triumphant scream, but knew it meant nothing — Sirius had only fallen through the archway, he would reappear from the other side any second….

But Sirius did not reappear.

"SIRIUS!" Harry yelled, "SIRIUS!"
Harry's breath was coming in searing gasps. Sirius must be just behind the curtain, he, Harry, would pull him back out again….

But as he sprinted toward the dais, Lupin grabbed Harry around the crest, holding him back.

"There's nothing you can do, Harry —"

"Get him, save him, he's only just gone through!"

"It's too late Harry —"

"We can still reach him —"

Harry struggled hard and viciously, but Lupin would not let go….

"There's nothing you can do Harry… nothing….He's gone."

He hasn't gone!" Harry yelled.

He did not believe it, he would not believe it; still he fought Lupin with every bit of strength he had: Lupin did not understand, people hid behind that curtain, he had heard them whispering the first time he had entered the room — Sirius was hiding, simply lurking out of sight —

"SIRIUS!" he bellowed, "SIRIUS!"

"He can't come back, Harry," said Lupin, his voice breaking as he struggled to contain Harry. "He can't come back, because he's d —"

Lupin was cut off by a new voice, one with an American accent, "Hey, um… Does this guy belong to anyone?"

The sounds of fighting ended abruptly as everyone turned to the source of the echoing voice: a boy about Harry's age with white hair purer than snow and killing curse-green eyes, surrounded by a soft white aura; floating above the stone floor.

Several noises of surprise and a cry of outrage could be heard as those present realized who the being's companion was.

"Sirius!" Harry cried out in relief upon seeing the familiar malnourished — if slightly nauseous — face of his dogfather. He flung himself at the man, disregarding the fact that there were Death Eaters all around them.

"FIENDFYRE!" Harry turned, ready to cast a 'protego' around himself and his godfather, and froze; a transparent green dome had appeared over them.

An instant later the dome had dissipated and Bellatrix was partially encased in ice. The white-haired being snatched her wand from her hand, "Tsk tsk. It's not very nice to attack someone when their back is turned."

SNAP! He smoothly broke the wand in half, ignoring Bellatrix's muffled shriek of rage.

Several of the remaining Death Eaters shook off their shock and began firing curses at the glowing being, forgetting about the Order members they had previously been battling.

Harry cried out a warning as a jet of green light shot toward Sirius' savior. The boy turned, the spell hitting him square on the nose….

The being sneezed. "Was that supposed to do something?"

Sirius cast a 'finite' on Neville's legs, cancelling the jinx as Harry hauled him up, and the three of them fled the room, still shocked by what had occurred.