I've had this idea in my head for a while, and I wrote it. Then I rewrote it. Then I rewrote it. It took some time, but here's some insight into what Narcissa Malfoy was thinking when she said Harry was dead. I'm kind of surprised there isn't more writing like this.
All dialogue and characters belong to J. K. Rowling.
When Narcissa thinks about it, she has always been pretending. She pretended to be a typical blonde flirt when she really would have been a Ravenclaw, if not for her self-preservation, cunning, and heritage. She pretended not to care when Andromeda left. She pretended to love Lucius at the beginning of their marriage. She pretended to let Draco grow up as his father did. She has to think about that to keep her sane.
She doesn't like to think about it, though, because Bellatrix is so skilled in Legilimency and she and her Lord will know immediately if her thoughts stray. All Narcissa is, Malfoy or Black, is a backdrop for the true people, the unpretenders who are more important, perfect and pure and meant to be somewhere else.
Narcissa waits as part of the scenery, an extra playing a supporter, a pale angel in a cloak of black fabric. She had wanted to rip the cloak up ever since she had found out Bellatrix had given it as a birthday present, but she is first and foremost a Black, and Blacks never show emotion or weakness.
The forest is cold and dangerous, but she doesn't dare shiver for fear Lucius or Bellatrix will glance over. Narcissa hears her sister's breath hitch and she looks up to see Yaxley and Dolohov without Potter.
"No sign of him, my Lord."
The Dark Lord, his face pale as death and just as cruel, runs his fingers over the wand she knows he didn't have at Malfoy Manor. He seems not to have anything to say, and Narcissa is glad of it, because a truth only Andromeda could have spoken is ringing in her ears.
Years ago, the Black sisters had gone to see the Dark Lord speak, back when he was still known in some parts as Tom. Narcissa had agreed with his ideas, though his proposed methods seemed extreme.
Bellatrix had been enthralled, hanging on every word the too-pale, handsome man had spoken to the point of taking it as gospel. Andromeda hadn't made any comment on it at the time, but afterwards, she had whispered, "Doesn't his voice give you a shivery feeling, like nails on a chalkboard?"
Now that handsome man's face is like chalkdust and he had killed thousands without blinking, and Bellatrix still has an obsessive fascination with him and his ideas, and Andromeda was a traitor, and Narcissa has stayed still, loyal as she listens.
But it is her sister who speaks. "My Lord -"
A skeletal hand raises and Bellatrix's mouth immediately closes. Narcissa feels strangle numb as she realises he's about to speak.
"I thought he would come." Narcissa has gotten used to the cold, clear voice filling her home that she barely notices Andromeda's shivery feeling anymore. "I expected him to come."
A fatal silence descends upon the clearing, and Narcissa wonders horribly if what she sees in the shadows is a spider's corpse before remembering that Acromantula eat their dead. The fire lighting them all in the worst way, he speaks again.
"I was, it seems... mistaken."
"You weren't."
Harry Potter, the Chosen One, the Boy Who Lived, the wizarding world's best and last hope, is standing there. It strikes Narcissa suddenly that he is Draco's age and walking to his own death, and her gasp is lost in the crowd's yells and sniggers.
He looks almost like his father, but his father never had those eyes, or that nose, or that look on his face. He is absolutely determined, no matter that Narcissa reads his fear in the way his mouth tightens and his hands shake under his sleeve.
He is too pale, his hair overlong, glasses frames singed, clothes torn, and he doesn't have his wand out as he takes a few deliberate steps towards the fire and the Dark Lord in an intended last stand. Potter wants to die with dignity, unlike his Mudblood mother, and Narcissa can respect that if nothing else, even if he is her son's age.
"HARRY! NO!"
Evidently surprised, Potter turns to see the half-breed Hagrid bound to one of the thicker trees and looking terrified for this seventeen-year-old's life. Narcissa wonders if anyone wound object to her death like that, or at all. She has an awful feeling that neither Draco nor Lucius would.
"NO!" He shouts. "NO! HARRY, WHAT'RE YEH -"
"QUIET!" snaps the one nearest him she knows is a half-blood called Thorfin Rowle. He casts a silencing spell that, surprisingly, works.
Narcissa hadn't noticed her dark sister beside her stand up, but she watches the scene avidly. Potter is determined to die this way, for all the people they would kill regardless. She suddenly feels so guilty she can hardly keep from crying, because she only sees what she has done wrong after it is too late. Her son will die because of her.
They stare at each other, fiercely calm, both knowing that everything hinges on what happens in this moment. Narcissa is witnessing history at its worst.
"Harry Potter, the boy who lived," the Dark Lord whispers through dead, smirking lips. She sees Potter begin to tremble and wishes her Lord were merciful enough to finish him.
He is, spidery fingers flicking the wand and vivid green hitting Potter straight in the heart. Narcissa knows he is dead now, but the Dark Lord crumples and hits the ground at the exact same moment that Potter does. Her hair flies through the dark air as her head snaps around to look at him. She is sure her grey eyes are wide open.
Bellatrix is beside him, touching his hand and trying to tell him wordlessly that she is forever loyal. "My Lord," she whispers through breathless lips. "My Lord..."
"That will do," he says coolly, standing up, red slit-eyes darting to Potter's corpse, unsure. She wonders what went wrong and dares to hope for mercy on her and her family.
Bellatrix stands by his side, pale and proud, as the Dark Lord straightens up. "I do not require assistance. The boy..." All eyes turn to the body sprawled on the forest floor. "Is he dead?"
Nobody is stupid enough to speak, but Narcissa's grey eyes widen and flick to Lucius. The Dark Lord notices this with a smirk and turns to her.
"You."
A small jab with his wand, and a burn mark appears on her arm. She lets out a half-scream unbecoming to her family and wishes she had never been so stupid as to join his forces.
"Examine him," the Dark Lord elaborates, a small smirk curling his lips, an odd gleam in his red eyes. "Tell me whether he is dead."
She stands up, nervous but determined not to show it. Her hands tremble, but she walks across to where Potter is lying. Narcissa's cloak makes an awful slithering sound trailing on the grass and dead leaves that remind her that everyone is watching. She wonders, in the back of her mind, whether the feeling in her stomach could be called stage fright if she has something to fear more.
He is alive.
Narcissa's eyes are most likely popping out of their sockets. Her pulse is as fearful as the boy's in front of her. It isn't the Dark Lord who makes the decision of wizarding history now, it is all up to her and her actions now.
"Is Draco alive?" she whispers, hiding her lips from the crowd of Death Eaters. If her answer was no, she would kill him. "Is he in the castle?"
"Yes," Potter tells her in a breath that could been wind. Narcissa's heart beats frantically as she stands up, faces the most brutal people she has ever met, and announces, "He is dead!"
They cheer, and she fears for her life.