Disclaimer: Rowling? Yes. JK!
She bends down. Her heart thuds. Miraculously, so does his. A continuous thrum pulses against her fingers, strong and unmistakable. Impossible, she thinks to herself - there is no recorded case of anybody ever surviving Avada Kedavra. And yet - Harry Potter is not just anybody. She smiles mirthlessly, wanting to laugh - but there is no laughter left in her, only fear. Fear for Draco - her darling boy, her one joy - Draco must be found. Draco is the priority.
"Is Draco in the castle?" she breathes, careful to shield the movement of her lips with her long blonde curtain of now grimy, matted hair.
Behind her she hears the Death Eaters shift impatiently. Then the murmurings die down into an implacable stillness. Hushed, they all crane forward for a glimpse of the Boy-Who-Lived. They wait, filled with anticipation, for her response. She waits, filled with trepidation, for his.
She nearly misses it when it comes - an almost imperceptible exhale, "yes". Her heart soars.
"I must prove you dead*", she whispers, willing him to trust her. Another slight nod. She moves so her hands are clearly visible and deliberately pierces him with her long nails. He is good, she grants that - there is no twitching, no flinching to give away what she has done.
And she has done what she has to.
Because for Narcissa, family has always come first. Family is everything.
Family is why she married Lucius.
Family is why she followed the Dark Lord.
And today, family is why she will betray him.
She rises to her feet. She lifts her head. Her eyes shine. She parts her lips and tells them.
"He is dead!"
*non-canon