After Harry has "disappeared", revealing that he was not as dead as he seemed, I had the feeling Bella would be likely to turn on her sister for having committed obvious treason (or whatever she'd call it in her Queen Death Eater-y vocabulary). Wrote this very quickly as a sort of "missing scene" that could have happened in the melee after without being particularly concerned whether it completely fits canonically or not.
At any rate, I hope you enjoy it if you're reading! :)
"LIAR!"
She was dodging curses and hexes as deftly as her sister was throwing them.
"YOU LIED TO HIM! YOU LIED TO THE DARK LORD, NARCISSA!"
It seemed the truth was out, but between the battle cries and dying screams surrounding them, Narcissa wasn't sure anybody had actually heard her sister's impassioned accusations.
"I had no choice!" she threw back over her shoulder, along with a harmless hex. No matter the narrowly-escaped killing curses, she couldn't bring herself to harm her sister. "Draco is still alive because of–"
"Don't you dare say because of Potter," Bellatrix spat. "How could you?!"
Brought to her knees by a jet of white light, Narcissa struggled to retaliate.
"You have betrayed Him, Narcissa, and he knows. You have betrayedme,your own sister!"
"No, Bella," the younger ground out as she tried to keep her dignity and not writhe on the floor in pain, "I did what was necessary. I have never betrayed you."
The spell lifted suddenly, yet before she could react another jet of light, red this time, had sent her flying back several feet to a hard landing on the flagstone floor. Wincing, Narcissa struggled to obtain her wand and fired off a few stunners in rapid succession. Blocked, all, by her elder sister's shields.
Cursing under her breath, Narcissa forced herself to stand and the two recommenced battle. She knew that her son and husband were safe, at the very least; it would make dying less difficult, knowing that each would still have the other.
A jet of green light whizzed past her ear, and Narcissa ducked aside just in time to receive the business end of an Expelliarmus intended for somebody else. Her eyes followed her wand through the air and tracked its final resting place to an area several feet away. She dodged Bellatrix's onslaught by weaving through the crowds, silently hoping that none who did not deserve it would pay for her evasion tactics.
Though she reached the area she could not find her wand, and assuming somebody had picked it up to defend themselves, turned once more to confront her attacker. The latter tore through the crowd, face contorted in the purest form of rage.
Their battle waged ever on, Narcissa now resorting only to wandless shields.
"This is enough, Bellatrix!"
"I have to agree," a deep voice resonated behind her, and she saw as a familiar hand and wand motioned in her peripheral.
Her sister was launched back, stopping only when she met with a wall.
Narcissa turned to face her saviour, remarking, "I had the situation handled."
"So it would appear," Lucius Malfoy drawled, a hint of a smile lighting up his tired eyes.
"Thank you," she replied softly as his arm draped about her shoulders, already escorting her hastily back out of the fiercest part of the battle zone.
"Draco–?"
"–is safe," he affirmed, gesturing to where their boy stood in a corner of the Great Hall, eyes haunted by the horror unfolding around him.
"Then let us tarry here no longer."