It hurt. It hurt almost more than anything John had ever lived through. That wrenching, that tearing apart. Removing one’s own wing was akin to removing part of one’s soul.
But as Sherlock lay there, silent and vulnerable and unconscious, the thread of life fluttering so faintly within him, there was no question in John’s mind. He knew this was the right thing to do.
Gently, carefully, he severed the wing and with an impossibly steady hand, he sliced Sherlock’s back open, just to the right of his spine. With skilled surgeon’s hands and angel’s magic, he transplanted the wing, reattaching bone and vein and tendon and sinew. The whole time, Sherlock’s breath was faint against John’s thigh, but mercifully steady.
After what felt like years, the work was done, and John cradled Sherlock in his hands, waiting to find out if it would take. Eventually, Sherlock’s face nuzzled sleepily against John’s thigh, and the wing fluttered and stretched, his body instinctively testing the bonds and connections. John let out a shuddering breath and stroked the soft curls at Sherlock’s nape.
"John?" Sherlock’s voice was muffled with sleep and trauma. "My back feels odd."
"Shh, Sherlock. You fell, you hurt yourself. I’ve taken care of it. Sleep for now."
Pliant and obliging for the first time in his life, Sherlock nodded off again, and John’s heart-rate settled for the first time since seeing him jump.