I know this was meant to be a one shot, but then the plot bunnies hit me, and people asked for some of the flash back scenes, so I complied.
Obviously a lot of this is borrowed from JK Rowling, but I hope you enjoy my additions to it.
I, like the BBC, apologise in advance for the feels.
They were back in Dumbledore's office, the windows dark, and Fawkes sat silent as Sherlock sat quite still, as Dumbledore walked around him, talking.
"the boy must not know, not until the last moment, not until it is necessary, otherwise how could he have the strength to do what must be done?"
"But what must he do?" Sherlock questioned still not seeing the point.
"That is between him and me. Now listen closely, Sherlock. There will come a time, after my death, do not argue, do not interrupt! (Sherlock had opened his mouth to speak)There will come a time when Lord Voldemort will seem to fear for the life of his snake."
"For Nagini?" Sherlock asked. He knew the snake was a horcrux, but why would Voldemort worry? He thought no one knew of the existence of his horcrux.
"Precisely. If there comes a time when Lord Voldemort stops sending that snake forth to do his bidding, but keeps it safe beside him under magical protection, then, I think, it will be safe to tell him."
"Tell him what?"
Dumbledore took a deep breath and closed his eyes. Sherlock was suddenly wary.
"Tell him that on the night Lord Voldemort tried to kill him, when poor Mr Watson cast his own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort's soul was blasted apart from the whole, and latched itself onto the only living soul left in that collapsed building. Part of Lord Voldemort lives inside the boy, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort's mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to and protected by the boy, Lord Voldemort cannot die."
Sherlock listened to Dumbledore from one end of a long tunnel; he seemed so far away from him, his voice echoing strangely in his ears.
"So the boy...the boy must die?" asked Sherlock quite calmly.
"And Voldemort himself must do it, Sherlock. That is essential."
Another long silence. Then Sherlock said, "I thought...all those years...that we were protecting him."
"We have protected him because it has been essential to teach him, to raise him, to let him try his strength," said Dumbledore, his eyes still tight shut. "Meanwhile, the connection between them grows ever stronger, a parasitic growth. Sometimes I have thought he suspects it himself. If I know him, he will have arranged matters so that when he does set out to meet his death, it will truly mean the end of Voldemort."
Dumbledore opened his eyes. Sherlock kept his expression neutral.
"You have kept him alive so that he can die at the right moment?" he plaintively
"Don't be shocked, Sherlock. How many men and women have you watched die?"
Sherlock didn't answer. He stood up. "You have used me."
"Meaning?" Dumbledore inquired
"I have spied for you and lied for you, put myself in mortal danger for you. Everything was supposed to be to keep the boy safe. Now you tell me you have been raising him like a pig for slaughter "
"But this is touching, Sherlock," said Dumbledore seriously. "Have you grown to care for the boy, after all?"
"For him?" roared Sherlock. "Expecto Patronum!"
From the tip of his wand burst a small spiky creature. It nuzzled itself against Sherlock's cheek then landed on the office floor, bounded once across the office, and soared out of the window. Dumbledore watched the hedgehog fly away, and as its silvery glow faded he turned back to Sherlock, and his eyes were full of tears, and his expression one of horror.
"I thought…Mary…"
"Mary meant nothing to me. It was him."
"Even after all this time?"
"Always," said Sherlock.
A single tear trickled down his face, right across the spot where the patronus had touched him.
