All Harry knew was that he couldn't let it happen again.
He crossed the room with a purpose, intending to fight right next to the Veil, but not at an angle at which he could fall through. That way, should he see someone getting too close or ready to slip, he could save him or her…probably sacrificing himself in the process. It was a faulty plan, at best, but it was better than letting the next victim fall through to their death while he stood by and watched, helpless. The sorrow he'd originally felt at Sirius' death was ebbing, replaced with even more anger at the Death Eaters and a fierce protectiveness for everyone around him.
He'd gotten them all into this mess, and he would die getting them out if he had to.
He threw curses at random Death Eaters as he crossed the room, trying desperately to end the battle before it got any worse. When he reached his destination, he ignored the alluring voices tempting him to cross through the Veil and continued fighting, feeling ridiculously outnumbered.
And then he locked eyes with Luna.
She was fighting a Death Eater he recognized and hated, and his stomach immediately boiled. Lucius Malfoy was throwing every curse known to man at Luna, and she was dancing and blocking, having very little chance of throwing an offensive spell back. A hundred plans ran through his mind. He couldn't yell at Luna: that would distract her and probably get her killed. He couldn't yell at Lucius: he would just get himself cursed, or Lucius would finally throw a Killing curse at Luna before turning to him. But she was getting dangerously close to the Veil…
Blocking a curse from behind, he threw a Stunning spell at Lucius, which missed, but distracted him momentarily, allowing Luna to cast another Stunning spell. Unfortunately, hers also missed, and Lucius sneered at the girl before brandishing his wand again. "Avada Kedavra!"
"Luna!" shouted Harry.
A jet of green light shot from Lucius' wand, and she twisted, missing the curse by mere centimeters. She looked terrified, and Harry realized somewhere in the back of his mind that it was the first time that he had ever seen her look that way. She took a step back…
Harry was running without seeing what was in front of him, desperate to save her from the same fate Sirius had encountered. The battle seemed to have fallen silent, noticing Luna's struggle and providing a timelier soundtrack. He was running faster than he ever had in his life…but she was falling so quickly…
He reached the Veil and, without really thinking before he acted, he threw his hand out and pulled her back, the effort of which threw them both backward. Luna landed directly on top of Harry, knocking the breath out of him.
She blinked once before falling off of him. "Oh. Harry." She was starting to form more words when her eyes shut and she passed out, and Harry tried frantically to wake her up.
And if I die
See you won't be so close to me
And I won't be the one
Who sticks around
"If I Die" Something Corporate