Preface: Harry, presumed dead, is hiding under the invisibility cloak and trying to find his way to Voldemort in the Battle of Hogwarts.
Within the center of of the Great Hall Bellatrix was still fighting too, fifty yards away from Voldemort, and like her master she dueled three at once: Hermione, Ginny, and Luna, all battling their hardest, but Bellatrix was equal to them, and Harry's attention was diverted as a Killing Curse shot so close to Ginny that she missed death by an inch –
He changed course, running at Bellatrix rather than Voldemort, but before he had gone a few steps he was knocked sideways.
"NOT MY FRIENDS YOU BITCH!"
Neville ran towards Bellatrix, rolling up his sleeves as he prepared to fire a spell. Bellatrix spun on the spot, roaring with laughter at the sight of her new challenger.
"STAY BACK!" Neville shouted to the three girls and with a swipe of his wand he began to duel. Harry watched with terror and elation as Neville's wand slashed and spun, and Bellatrix Lestrange's smile faltered and became a snarl. Jets of light flew from both wands and the floor beneath the two duelers became cracked and hot; both were fighting to kill.
"NO!" Neville cried as a few D.A. members ran forward, trying to come to his aid, "Let me have her –SHE BELONGS TO ME!"
Hundreds of people now lined the walls, watching the two fights. Voldemort dueled his three opponents and Neville fought Bellatrix. Harry stood, invisible, torn between both fights, wanting to attack and yet to protect, unable to be sure he would not hit the innocent.
"Do your parents remember me, Neville?" Bellatrix taunted, as mad as her master, capering as Neville's spells and curses danced around her, "Do they ever talk about the last face they saw before they forgot their own identities?"
"You will never –touch –another –friend –" Neville growled as he deflected her blows and shot curses of his own.
"Do they remember you, Neville?" Bellatrix cackled, spinning on her heel as jets of light blasted the ground around her feet, just missing their dancing target, "Do they even recognize you when you visit them?"
"—Never –harm—another—family—" Neville snarled, eyes alight with a fire Harry had never seen before.
"I'm told St. Mungos now keeps them as permanent residents. Maybe when I'm done with you I'll go and end their pitiful existences?" Bellatrix shrieked and a fire of green light shot from her wand, burning the edge of Neville's coat sleeve.
" –AGAIN!" Neville bellowed.
Bellatrix laughed, the same exhilarated laugh her cousin Sirius had given as he topple through the veil, and suddenly Harry knew what was going to happen before it did.
Neville's curse soared beneath Bellatrix's outstretched arm, hitting her squarely in the chest. Her eyes bulged, smile frozen on her lips, and for the tiniest space of time she knew what had happened, and then she toppled.
The watching crowd roared, Voldemort screamed, and Neville grinned in triumph.