Bruce turned around just in time to have a stout lacquered box thrust into his hands. Elegant paintings of a fiery bird with its wings wrapped around the corners gleamed on its shiny surface.
"Natasha... what's this?" he asked. It was oddly heavy, and strangely cool in his hands. And somehow the box was vibrating ever-so-slightly. It felt like holding an unexploded bomb.
She stared at him for a long minute, her expression subtly shifting. He wasn't sure if that twitch of an eyebrow or quirk of her lip meant amusement or fear or uncertainty, but the fact that she was emoting at all meant whatever it was, it was serious.
"It's for you. I want you to have it. Just in case."
There was something in her tone that Bruce thought he understood, a sort of acceptance. The same kind of acceptance he'd felt the night he'd held a gun to his mouth.
"Your will?" he asked quietly.
Her lips quirked again, and Bruce thought he saw something between amusement and sorrow.
"My living will. If anything ever goes wrong." She paused and thought that over. "If I ever go wrong."
"You understand why. You're the only one on this team who does. Sometimes there's a line, and when you cross it, there's no going back. You can't afford not to cover that possibility. So I'm trusting you to do this for me if you have to."
Bruce clutched the box hard, and felt the vibration get more vigorous under his fingers as the color rose in Natasha's cheeks.
"What's in here, Natasha?"
She took a step back from him and drew one of the knives hidden at her belt.
"I've had my brain played with in ways most people can't imagine. I have red... so much red in my ledger. Someday it's just not going to balance out." She took a deep breath and let it out again slowly. "I'm sixty-eight years old, Bruce. I did something a long time ago so I'd never die, but I didn't stop to think about what that could mean."
Bruce suddenly got a chill of premonition, but he didn't quite move fast enough to prevent Natasha from driving the blade into her heart.
"Natasha!" He leapt for her, cradling her as she dropped to the floor, his hand over the hilt, afraid to pull it out, terrified to leave it in.
"It's ok," she rasped, and put her hand on the hilt next to his. There was blood on her lips and gurgling in her lungs, and Bruce could feel the Hulk roaring in impotent fear as she looked up at him, so pale. "It's ok." He wanted to stop her as she drew the blade from her heart, red and slick, but just held her instead. Held her as her breaths deepened, the gurgling stopped, and her head came up.
Good as new. Just as Bruce had been the night the Hulk spit a bullet back into his hand.
She took his hand and put it on the box beside him. "It's my heart. That's the only thing that can ever kill me. If you need to, open up the box, crack the egg inside, and smash the diamond. Then I can die."
Bruce could feel the box vibrating, pulsing, fluttering under his fingers like the quivering of a nervous heart. Inside him the Hulk raised his head at the key word of “smash,” and Bruce felt a wordless assent at Natasha’s request.
"Don't scare me like that again," he said, and held her close to him with his other arm, heedless of the blood.
"But you'll do it?" she asked insistently.
Do you understand? That was what she was really asking.
He tilted his head down and she met him, kissing him soft and slow, spiced with saltwater from tears.
“Yes.” The answer to both of her questions.
The box thumped madly with relief under his hand, the same one with which he’d caught a bullet, alive and vibrant as the lover in his arms. And fragile as a diamond.