Note: I own nothing but my own plot, everything else is the BBC's, Stephen Moffat/Mark Gatiss's, Arthur Conan Doyle's, or J.K. Rowling's. I just like to play here. Not beta'd or Brit-picked. This is a sequel to "Like a Phoenix," and "Phoenix Rising," so if you haven't read them, you probably should. This ties in with the BBC Sherlock storyline, not the original ACD canon, and I've tweaked the timeline a few years to make John the same age as Harry, Ron, and Hermione.

Now—as to this one? I'll say right here that I thought the entire storyline with Eurus in season four was ridiculous, but still . . . that island sure did look like Azkaban, and it would explain a lot if she were a witch, so . . . This is Part Three of the Sherlock/Harry Potter crossover that started with "Like a Phoenix." I'm ignoring all the parts of the Final Problem that I thought were absurd. (That's most of it, honestly. She flitted to and fro from that top security island fortress way too often, way too easily. She was far too murderous to have that turn-around at the end make any sense—a hug from her big brother made it all better?) Instead, I'm just taking that interesting coincidence of Sherlock having a sister locked away on a secure island in the North Atlantic, combining it with this crossover of mine, and going with it. Enjoy!

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Sherlock felt one of his eyebrows rise as he opened the door to see an impatient wizard standing on his step.

"Harry," he said with some surprise. "John is not here."

The man nodded. "I know. I'm here to see you."

"Well, then please come in."

Sherlock stepped back, opening the door wider to let Harry Potter come inside. The wizard was moving with a focused energy, like a coiled spring—contained but ready to explode if he relaxed that control even a little.

"How may I help you today?"

Harry looked at him, eyes burning. "Why did you never tell me you had a sister?"

Sherlock felt the words like a blow.

"I don't."

Harry shook his head. "Don't lie to me, Sherlock. Having a witch in the family changes everything. The statute of secrecy doesn't apply, which means you have to have known—about all of it. All those years of playing dumb. Why didn't you say?"

"Harry, I don't know what you're talking about. The only sibling I have, thank God, is Mycroft. I'm sure I would never have survived a second one."

He stopped pacing. "You're trying to tell me that Eurus Holmes is not your sister?"

The name was not familiar, but it sent a thrill up Sherlock's spine like it should be familiar.

"I have to confess that it sounds like a name my parents would have chosen, but, no. To my knowledge I've never had a sister."

Instead of settling Harry, this made him suddenly turn on Sherlock, stepping closer, his eyes intent as he peered into his Sherlock's.

"Harry, what are you …?"

"Have you ever been obliviated?" Harry asked, eyes searching. "I mean, other than Mary?"

"That is such a nonsense word," Sherlock said, feeling more off-balance than he liked, trying not to retreat, but pushed back by the heat of Harry's intensity. "Why would my memory be modified?"

The very thought was abhorrent. That someone could reach into his head and alter his mind palace was unthinkable. Even as he spoke, he was prodding at the walls, looking for hidden doorways, missing passageways.

If he had had a sister and had been forced to forget her, that would have to be an entire wing, surely?

Harry plopped down in John's seat, and pushed his hand through his riotous hair. "Because you have the most impressive memory of anyone I've ever seen, and I can't imagine you forgetting this. You, of all people! But it's the only answer. This woman has to be your sister."

He pulled a photo out of his pocket and handed it over.

Staring at the unfamiliar face, Sherlock had to admit, there were similarities. "What does Mycroft say?"

"I haven't asked him. I came to you first."

Trying to force his brain into productive thought, Sherlock studied the photo, awed as always at the way wizarding pictures moved. "You say you only learned of her today. This is in connection to your job as Head Auror?"

Another nod. "I had to visit Azkaban prison today, and … well, it turns out there is a building used not for prisoners but …"

"The mentally ill," Sherlock finished for him, noting the lines of hidden strain on the woman's forehead. "Including this woman?"

"Yes. The highest of high security. It shouldn't surprise you that we don't exactly have mental healthcare in the wizarding world. Those who have … issues … tend to be taken care of at home or just ignored unless they do something truly criminal. However, there are a few—very few—cases where they are locked away for their own good."

"And this … Eurus … is one of those cases?"

"Exactly. Even without a wand, without training … she is remarkable. Very much a danger to herself and others. So much so, that they felt they had no choice but to isolate her entirely."

Iso … When had the room gotten so cold?

"At what age?"

"Young." Harry looked like he wanted to be on his feet again. "I've come to accept that much about the wizarding world is backward, in relation to the non-magic world, but knowing your family? This …"

"It shocks you," Sherlock said, trying not to think of his own shock.

"Yes. Your family is well off, well-connected, with all available resources. And you are all so gifted at solving impossible problems." Harry pushed back his hair again, a sure sign of nerves for a man so well-acquainted with stressful conversations. "So what must have happened to drive your parents to lock away their daughter—and presumably alter your memory at the same time?"

"Something … terrible."

As he said it, Sherlock could feel a pressure in his head, as if someone was knocking at a wall, seeking entrance … or an escape.

"You said Azkaban?"

"The wizarding prison. It used to be guarded by dementors which … honestly, I can't imagine what they would do near anyone with mental health issues. They forced a person to relive their worst memories, forget all happiness. They regularly drove people mad. I can only hope they were kept distant from the building housing people like your sister."

Watching his face, Sherlock recognized the signs of a person trying to fool themselves. "But you don't believe that."

"No, I don't." Harry was on his feet, not quite pacing, but clearly unable to sit still. "I don't know what to make of this. To be at Azkaban, she had to be a witch, a powerful one, but you didn't know about magic until you found out about John a few years ago. Neither did Mycroft. How is that possible? Unless … do your parents know, do you think?"

Sherlock tried to imagine his parents—brilliant people in their own way, but not remotely observant—keeping any such secret from both him and Mycroft. (Because, say what you would about his overbearing brother, Mycroft was smart. Annoying, overbearing, and insufferable, yes, but definitely intelligent.) Perhaps their parents had unseen depths.

"I don't see how that's possible. A secret of that magnitude? But … would the wizarding world have been willing to obliviate an entire family?"

Surely that had to be unthinkable?

"Since you were just muggles? I don't think they would have hesitated, though …" Harry's voice trailed off as he thought. "Obliviation is usually for something specific, almost always something recent. If a non-magic person sees a wizard apparate right in front of them, for example, or they see a spell, or pick up a set of keys spelled to disappear … basic muggle-baiting. An obliviate spell is simple. That one memory is removed or redirected, and they go back to their regular lives, none the wiser. It's minor, like bandaging over a paper cut, or cleaning up a stain. A little scrubbing and your carpet is as good as new—you're not redecorating and recreating the entire rug. Instead of the witness worrying over their sanity or trying to figure out how something happened, they are given another memory, a rationale to believe instead."

"Simple misdirection."

"Exactly. It's easy enough for a single event, and when done responsibly, there's no harm." He shrugged. "There is some debate about whether multiple obliviations cause long-term damage. Hermione has a theory that the surging number of dementia cases are linked to the growing number of obliviations the ministry is having to perform these days—with so many cameras and recording devices. Wizards and witches aren't careful enough and are being seen too often. There is speculation that repeated memory spells affect the brain's capacity to record properly …"

"But that would be an exception, for someone to need multiple obliviations?"

"Definitely … though there were obviously more during the war … like when we had to make people forget that a dragon had flown over London. I think they used the filming-a-movie excuse for that one. I don't really know. That was the day we had the battle, and I wasn't really paying attention to the news."

"A dragon ...? No, never mind." Though that was a tangent he definitely was asking John about later! "Still … that's quite different from forgetting a … sister."

By the fireplace, Harry had the grace to look ashamed. "I know. I can't … I can't even imagine that. Family … The wizarding world has always considered that important. I can't imagine they would have just … taken your sister away. And … how do you block an entire person, their entire life, from a memory? That's not as simple as a single incident. That's presumably … years of intertwined memories. It's not like she was taken away as a newborn! I can't imagine how they would have dared …"

"Because we don't have magic and are therefore not important," Sherlock said, trying to keep the bitterness from his voice.

He laid his fingertips together, visualizing forging connections with each physical contact. "You say you were at Azkaban today, and that you hadn't realized there was a mental ward. What made you go there today?"

"There had been several odd blips in the wards. I'd call them glitches or bugs if they were a computer feed. I brought a team of wards experts to investigate and found that this entire building existed that I hadn't known about. We removed the dementors after the war and shored up all the wards, but this one building was so well protected, we didn't even see it. Not until today."

"And what happened?"

Harry looked at Sherlock, eyes firm. "The wards malfunction that drew our attention. It acted like … do you know what a Fidelius is?"

"The spell that hides a secret?"

"Yes." There was a moment of silence, then he continued, "After I arrived, I was looking at the records for the last time the wards had been upgraded, and found reference to some work that that had been done about thirty years ago. So I started cross-referencing and looked for patients that had arrived around that time—for anything that could explain it."

"And you found my sister."

"A patient named Eurus Holmes who had two brothers, both of whom were, quote, 'no longer in the picture.' She's been there since she was about five."

Sherlock was barely able to suppress a shudder. If this sister was anything like him or Mycroft, she must need mental stimuli, and being completely isolated since age five?

If she hadn't been mad before, she certainly would be crazy now.

He said as much, and Harry nodded. "That's what I'm afraid of. Going by the reports, she's a natural legilimens … what non-magicals call a mind-reader. She's kept isolated because her natural gift is to be able to get into a person's head and …"

"Manipulate them," Sherlock finished. He wondered how much simpler his life would be if he could just will people to do as he wanted them to do. No wasting time being polite and asking nicely. No idiotic people doing the wrong things all the time. They would just do what he needed. Simple.

Except … he liked to think he had learned a few things since Moriarty. He knew himself to be brilliant … well, he was … but he was also human. Given the comparison between the mad, psychopathic mastermind, and the occasionally infuriating but very decent, human John Watson, Sherlock would pick "boring human" any day.

But would he have chosen that when he was younger? He had been so sure that he knew best about … everything. And, honestly, wasn't that the primary conflict between himself and Mycroft? Both of them so certain they were right, always, and neither with a gift of compromise. Imagine with a younger sister, just as determined, added to the mix? He had a sudden image of a young girl stamping her foot in frustration. If he had been infuriated with Mycroft's condescension throughout his entire childhood (life), how much worse would it have been to be younger sibling to both of them?

Especially if said younger sister was a witch in a non-magical family.

How easy would it have been for her to manipulate all of them to get her own way? How dangerous?

As much as Sherlock hated to admit it, he was a better person for the limitations John had always insisted on—basic courtesy, an awareness that other people had needs, rights, desires … He might never be a warm, caring person, but Sherlock had grown to appreciate the good parts of humanity (even if the bad parts seemed more prevalent).

But Sherlock had had the advantage of growing up. He had gone to school, to university. He had made friends and enemies, and learned some harsh lessons about his own infallibility, along with the miraculous ones about the power of friendship.

If he had been locked away behind walls as a child and left alone …?

"Tell me."

"My best guess right now is that we missed seeing that building because … Eurus didn't want us to see it. The fact that there are … fluctuations … now is what worries me."

Racing though it was, Sherlock wasn't sure his brain could keep up with the calculations it was making.

"So … a young girl, mentally unstable, with the ability literally to influence the minds around her, was sent basically to solitary confinement on a prison island in the middle of the ocean—next to a prison filled with equally insane death eaters, and guards that eliminate all possibility of happiness?

Harry nodded. "That's what it looks like."

"Deliberate?"

"Unclear," Harry said. "I don't know what she knows, or if she is aware that you now know about the wizarding world."

"But I've known about magic for years—since before my … absence. Why now?" He took a sharper look at Harry's posture. "What aren't you telling me?"

"It's just speculation, but … the more I think about it, the more I suspect that you were never obliviated in the ordinary sense. I think that it was your sister that made you forget. Maybe she didn't want you to remember her being mad. Maybe she was hiding something. Maybe she just didn't want you to miss her. But now she's changed her mind."

"But why?"

"Maybe she's jealous and wants a resurrection of her own?"

"It's turning into something of a specialty, isn't it? Coming back from the dead, or near-dead. You, John, me …"

Naturally that was when his phone buzzed with a text telling him to turn on the telly, where he saw a face from his nightmares repeating "Did you miss me? Did you miss me? Did you miss me?"

He looked over at Harry. "I'm blaming you for this."

Harry looked like he wanted to protest, but just held up his hands. "The timing is interesting, isn't it?"

"That's one word for it," Sherlock said, grumbling as he typed a reply.

"I wonder if she was involved with Moriarty?"

"Involved?"

"Not romantically … or, Merlin, I hope not! But this coming up just as we are having this conversation?"

Hmm. That was a good point, and Sherlock didn't believe in coincidences.

"It looks like we're going to Azkaban."

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