I'm so sorry for the long wait! Here's the final chapter.

When Mycroft returned from a meeting, he found his brother waiting for him in his office. Sherlock did not greet him as he came in and sat down behind his desk in his chair. Mycroft arched an eyebrow at him in a silent question, but a deep scowl was the only thing he got in return. The older brother rolled his eyes in an obvious show of great suffering.

"Okay, I'll bite. Why are you here, Sherlock?"

"I remember now."

"Remember what, pray tell?"

"You. All through the night. Everything."

"Oh."

"Oh? That's all you got to say? Oh?"

"Well, what do you want me to say? It's all in the past, Sherlock. You remembering it or rewriting it doesn't really change a lot."

"It does!"

"Exactly what does it change?"

"Well, maybe the fact that you apparently have promised your death to our sister! Really, Mycroft, of all people I've never seen you as the suicidal type."

"Is that what this's about?" Mycroft asked. When Sherlock pursed his lips and refused to answer, he rolled his eyes for the second time. "Don't be so dramatic about it, Sherlock. You and your drama, really."

"So you're not denying it? Are you mad? You must've gone senile to promise her that! What in the bloody hell were you thinking?!" Sherlock roared.

"Sherlock. Death is inevitable for everyone. Eventually I'll die one way or the other, and frankly I have no real preference in that matter. Besides, you've seen her. Uncle Rudy and I have basically robbed her of any chance at normality. Her whole life has been snatched from her. At the very least she should be able to choose her own death."

A realization dawned on Sherlock.

"…She will be killed when you die."

Mycroft sighed.

"Yes. The only reason 'people upstairs' are letting her live despite the potential threat she poses against our nation is because I vouched for her. I promised them I could keep her in line and get her assistance when needed. So when I'm dead, there will be an immediate order to remove the unmonitored threat."

"So you'll just die whenever she feels like ending her life? What are you two, some sort of symbiotic soulmates?" Sherlock scoffed.

"You have no right to question my decision," Mycroft answered in a hushed voice. "You weren't there, Sherlock. You weren't there to see her grow up inside that facility. Imagine a little girl, confined in isolation and treated like a lab animal."

"No lab animal gets a Stradivarius."

"The point is, I am in no position to deny her this little request. Not after everything I've done to her."

"And what exactly have you done to her? It was her choice, Mycroft! It was her choice to kidnap and kill Victor, it was her choice to burn down Musgrave! May I remind you that I almost died in that fire? You didn't force her to do any of these things, why do you feel responsible for them? You're deluding yourself because you're still in denial that our sister is a psychopathic monster!"

"Sherlock!"

"Face it, Mycroft! She is a monster. And I'm not going to lose my brother to her," Sherlock stated fiercely. "Try all you might, but you're not dying, not on my watch." Sherlock's voice was pained. He knew the gravity of the promise, now more than ever. He was still haunted by the dying image of Mary Watson. But he was still making the vow, because he could not not do it.

"…Enough of this, Sherlock. I've got things to do, and I'm sure you've got things to blow up back in your flat."

"Well too bad, I'm not leaving till I hear you say it."

"Say what?"

"That you won't foolishly throw your life away at a call from Eurus."

"Sherlock."

"Say it."

"You are being childish—"

"Say. It."

Mycroft pursed his lips. He refused to speak anymore, his steely eyes glinting like they did when he refused to pull the trigger on the governor at Sherrinford.

Sherlock crossed his arms, equally defiant. "Fine, then I'll say it. You will not kill yourself or let yourself be killed, because then that will be the end of the Holmes line," he said.

"Sherlock," Mycroft said warningly.

"Just think about it rationally, Mycroft. Do you really think Eurus will willingly accept death when you do? Who do you think can possibly defeat Eurus after you're gone? Your security team? MI6? Please. They won't come even close to a match to Eurus. But if you die because of her, I promise you I'll dedicate my whole life to bring justice upon her. It will very likely be the demise of both of us. So, dear brother," Sherlock grinned a devilish grin, a pirate's grin, "stick to your diet and stay healthy. You need to outlive her and me, and you've already got a huge disadvantage in age."

Sherlock stood up without further ado and showed himself out. His whistling of the familiar tune of lullaby echoed through the quiet hallway.

Again, I'm so, so sorry for not updating for months! I'm really bad at finishing stories... in fact, I think this is actually the first multi-chapter fanfic that I saw through to the end. Thank you all for reading (and reviewing!) this story. I hope I will be able to bring you another story (not sure which fandom) soon!