(That's the woman in mask's ball gown in the photo, imagine it with a mask)

"You have a letter."Mrs.Hudson said giving the white envelope to Sherlock, who had just came into the flat after failing to question Veronica again because of her health conditions.
"Who brought this?"Sherlock asked examining the envelope carefully.
"A postman."Mrs. Hudson said.
"What is it?"John asked who had just gotten into the flat from work, after Sherlock.
Sherlock opened the envelope. There was a short letter in it.
"What a pitty that little Veronica isn't able to talk since she is the very person who knows my true identity. Mr. Holmes let's admit one thing. You can not find me unless I want you to. And how lucky you are because I want you to find me now."Sherlock read it out loud.
"What does that mean?"asked John.
"That's not all."Sherlock suddenly realized the invitation card in the envelope.
John's eyes widened when he saw the invitation card.
"Is she inviting you to the Sapphire Night?!"John said, surprised.
A smirk appeared on Sherlock's face.
"Obviously. She loves disguising. A masked ball. What a perfect place for her to be in."
"You know, it's the most expensive new year celebration ball in England."
"And also the biggest masquerade."Sherlock completed.
"Oh there is another note there."John said pointing the card.
Sherlock turned the invitation card.
"I've already sent the mask to you Mr.Holmes"
Sherlock eagerly smiled at the note.

30th of December~two weeks later
"Your name sir?"the man in front of the entrance of the great manor in the forest, asked with a list in his hand.
Prefer to be with his family. He needs this job. Doesn't like the music inside. Late forties.
"Sir?"he asked again, annoyed.
"Sherlock Holmes."
He quickly found his name on the list.
"Could you please take off your mask for a moment sir?"he asked before clarifying his true identity according to the photo above his name.
Sherlock took off his the phantom's mask. The man nodded and put a thick on his name.
"Welcome to the Sapphire Night, Mr.Holmes."he said and stood back in order to let Sherlock get inside.Sherlock put his mask back on when he got inside.
Inside the huge manor was mesmerising. Everything seemed to belong to an 18th century royal court.
There was a colerature soprano singing with the orchestra in the background. The beauty of the music was the first thing that caught Sherlock's attention. Different. Never heard before. Interesting. Very Good. He thought.
All the people were wearing masks. The tuxedos and dresses seemed quite expensive. Women's jewelleries were shining and they all looked very elegant.
"A drink sir?"
Sherlock was interrupted by a waiter.
"Thanks."he took one of the champagne glasses and walked throughout the enormous hall. He wondered the owner of the mansion. He or she must have been very rich. But of course he had a guess.
Sherlock felt starry eyes on him. Well, no one could blame those women because of this, since he was looking very attractive in his black tuxedo and with the phantom's mask on the left half of his face. He looked mysterious and all eyes were on him. However, he didn't care. He was waiting for his mysterious companion to arrive.
Shining, sparkling, very much alive and mesmerising. This was how the ball room looked like.
He was getting more and more impatient as the time goes by, on the other hand, the attention on him was getting higher.
The soprano sung three arias since he had first came in. And now he was drinking his third glass.
Suddenly the orchestra stopped. The ball room became silent in a moment and everyone was looking around to understand what was happening. Sherlock was getting more and more excited and nervous.
Suddenly the enormous door above the double stairs, opened. Everyone's attention turned the door above the orchestra, from the main entrance door. A shadow appeared in front of the door. Everyone was whispering something about the mysterious woman who was gently walking towards the stairs. As she was walking under the lights, her appearance was getting clear.
Sherlock knew she was the very woman who was being waited for to come.
She started to descend stairs. She was wearing a beautiful gown. Her dark red hair was perfectly bunned on her head. And she was wearing a mask. When she took the last step, the orchestra started to play.
Sherlock  stood still in the middle of the ball room, astonished. All eyes locked on her. Every movements were being watched. And when she started to walk towards Sherlock, all eyes suddenly, locked on him. Sherlock felt a ridicilous and a weird feeling inside him. He was so impressed that he even started to sweat. He couldn't control himself. He locked his eyes on her gorceous ice blue ones.
He saw her eyes sparkled. Her gaze was piercing and Sherlock just couldn't take off his eyes on her. She was completely mesmerising.
Without cutting the eye contact, she raised her hand and flicked. With this, the orchestra started to play a waltz.
He cleared his throat and under the curious and envious looks, he reached out her hand.
Her silk gloved hand took his, he put his other hand on her  waist and pulled her closer to him. The music took it's rythm and they started to dance.
A smirk widened on her dark red lips.
They just danced. And he realized how professional dancer she was. She was floating like an angel. Her movements were so naiive yet confident. He really wanted to see the face beneath the mask. But he was enchanted by she herself and the beautiful waltz music. It was different. Never before had he heard it.
He was unable to realize the other people's envious and admire filled looks.
He didn't realize they were the only couple that were dancing.
They were dancing in the middle of the ballroom, just under a great chandeliar. They were surrounded by other people. And also he, again, wasn't avare of the press taking hundreds of photos of them.
Dancing with her, just like floating with an angel. He felt incredibly different while he was holding her hand.
They looked right into each other's eyes, throughout the whole dance.
When finally the music finished, a huge applaus was heard. But they didn't care. They were busy with looking at each other's eyes.
She didn't let his hand go and she hold it tightly and headed him towards the terrace.
Outside the hall was cold. Extremely cold indeed. Her bare shoulders were freezing but she didn't care.
Sherlock looked around. The terrace was huge. The night was perfect and the stars were shining with all their mights. The manor was located in a high point. He could see London's lights shining very far away from them.
"At last we met Mr.Holmes."she was the first one who broke the awkward silence.
"It seems so."he said.
"the mask really suits you."she said pointing his mask.
"Thank you. Your's suits you too but I prefer not you to wear this on your face."
"I know it suits me. But I'm afraid I can not take it off Mr.Holmes. Isn't it enough to hear my real voice?"she asked.
"I know your face."Sherlock stated.
"But you don't know who I am."
"I'll find out soon."he said and stopped for a moment."give me a reason to not to arrest you right now."
She smiled confidently.
"I can give you plenty of. However, just one of them will be enough to explain myself."
"Oh, which one is it?"
She smiled again.
"You can not arrest me Mr.Holmes because you need me."she said. Sherlock raised his eyebrows.
"Do I?"
"Yes you do. Oh come on, you are dying to hear how and why have I done all these thefts, aren't you?"
Sherlock didn't respond.
"And you know, if you arrest me, you'll never know."she continued.
"And, if you wanted to arrest me, you would have called DI-who was he?- oh Lestrade the man who you always forget his name-. You didn't call him. I bet no one knows that you are here except your boyfriend, John."
"He is NOT my boyfriend."Sherlock hissed.
"Isn't he? Well be it than. Anyway, where was I? Oh, in conclusion you wanted to meet me and interrogate me alone. You didn't  think arresting me even for one second. Also you are still confused about the documents that Mycroft gave you.
"Tell me by the way, have you found out anything from that files yet?"she asked to very much impressed and astonished Sherlock.
"One of them is you. I'm sure of it."
She laughed slightly.
"You think so? I see. You really think that I'm a ghost don't you?"
Anger filled his eyes but he didn't respond. He knew she was enjoying incredibly much.
"I know everything Mr.Holmes. When I say everything, I mean it." Her eyes sparkled beneath her mask.
"Than let's make true your deductions."he said looking at her necklace. "Nice necklace. Seems very expensive."he said ironically.
"It wasn't though. It was a gift."she said gently touching her necklace.
"A gift from old Mrs.Morgan? How generous she is!"Sherlock said, deridingly.
"Tell me how have I done that one."she said leaning against to one of the huge columns behind her.
"It was obvious. You hacked the security system, drugged the security guards. Than you came in from the rooftop. You had the key for the door at the rooftop so it was easy to get in. Being extremely careful you found the room where the destiny stone was exhibited. You used diamond to destroy the glass. But of course you were disguised throughout the whole theft. You were wearing a black wig. But you put the red hair strand there in order to deceive us."
She interrupted him.
"How did you know I was wearing a black wig?"she asked confused.
Sherlock smiled.
"You weren't as careful as you think you were. I investigated The Cygne twice and in the second one, I was on the rooftop and guess what I found?"he asked.
She furrowed her eyebrows.
"A black hair strand and it was yours."he stated. She nodded smiling.
"Why didn't you tell the police?"she asked.
"I wanted to hear it from you first."
"Oh. Please do continue Mr.Holmes, I'm very impressed."
Sherlock smiled with contentment.
"You found a homeless drug addict woman body-you didn't find it you stole it from St.Bartholomew's morgue-and made her look like elisa day. You tried to deceive me with this trick, I have to admit you were partly succesful. You managed to baffle me."
"I managed it three times."she said.
He didn't say anything.
"And solving the case of Donna Shalott took you years."she reminded eagerly.
"Your turn. Why did you choose Veronica?"he asked.
"Veronica. The sniffy rich insolent girl. I had an issue with her. Scaring her to death was so enjoyable. Ah, you should have seen her. The fear, the fear of death. It was so clear in her eyes. She thought I was going to kill her."she laughed in disbelief.
"Who cares her poor soul!Seeing the true fear in her eyes was the biggest contentment I have ever felt. She has a quite awful sense of fashion by the way. My God that dress!"she sighed in disbelief.
"What kind of issue?"
"Kind of issue that doesn't concern you. And hear me telling, I'll continue to play with her. It's really enjoyable."she said.
"She'll talk."Sherlock said.
"No she won't Mr.Holmes. She is MY slave. She does whatever I want."
"Don't be so sure."
"Oh why is that?"
"Because she WILL talk. I'll make her talk."
"God bless you Mr.Holmes, you made me laugh."she laughed.
"I wonder how your face will be when I arrest you?"
"Really good question. But in order to find the answer, first you need to see my face don't you?"she asked. Sherlock smirked.
"Oh please, it's not important because I know your face."
She looked confuse.
"Do you?"
"Yes I do. The only reason I'm not telling the others is because first I want your strings in my hands. After that they can arrest you. Also, it's too early to arrest you."
"Mr. Holmes, you are a dreamer. And I don't think you are as clever as you think your are. If you weree that clever, you wouldn't think that you could catch me."
"Why not"
"Because it's impossible. If you like you can arrest me now. But I can always escape."
"I can describe you. I can start a international investigation"Sherlock spoke angrily.
"If only you would know how do I look"she said melodically which made Sherlock angrier. Sherlock happened to talk but was interrupted by her.
"Mr. Holmes, even if you know my face, you can not clarify my identity. Because there is NO REAL ME. Because real me is DEAD. I am dead right now. Therefore I can be ANYONE. Even you!"
Her harsh explanation was enough to let Sherlock know his failure.
"I'm telling you these because I don't want to wait for you to solve these. The process is rather enjoyable but still, waste of time. I want to see you fail!"
"The actual dreamer is you."Sherlock hissed. They both started to shout at each other.
"You don't know who I am."
"Oh really?!"she said deridingly. "You are Sherlock Holmes. You claim that you are the most clever human being in the world which you are mistaken. You think sentiments are chemical defects and that's why you avoid showing any kind of sentiments. You see yourself quite strong but you are not. You are damaged. You have weaknesses and your biggest weakness is your sentiments. Hiding them doesn't work Mr.Holmes. You are forgetting something. Sentiments don't make you weak. They make you stronger. That's why you are not strong. You hide your feelings because you don't know how to control them. Now, do tell me I am wrong."
His blood was boiling with pure exasperation.
"Let me tell you my deductions. You are an insolent damaged little girl. You are a coward. You see yourself as the ruler of the darkness but you don't rule anything. You are it's slave. It controls you. You are not strong because you let your sentiments get better on you. It is a mistake. It makes you vivid. You just want to take revenge but why? An old issues? Most likely but I'll find out and I will explain every single them at the court while I'm looking at your eyes."'he finished.
"So, we know each other quite well don't we?"she said calmly, smiling. The storm suddenly settled.
"It seems so."Sherlock nodded.
Her confident and deriding expression suddenly changed.
"Do you hear the song playing?"she said. Sherlock paid attention to the song playing inside.
"Intorno all'idol mio"
The soprano's voice was heard.
"My favorite."she stated, looking away from him. He realized the sudden change on her.
"Spirate pur, spirate aure
The soprano's voice came higher.
"Soavi e grate;"
"It's from Orontea."Sherlock suddenly said.
"Yes."she closed her eyes.
"Can you feel the beauty of it? /The notes are floating in the air. /Gently touching my ear. /In their hearts, they never carry the fear. /And in the end, they get in their lair. /The lair of my heart. /And the sadness of my eyes, consumes it's light. Ah, you can not believe how much this song inspires me?!"
Sherlock stood still, he was speechless. He couldn't control his brain. Now, the control is in his heart and it wasn't good.
"I used to sing this song. In my dark times. It would give me hope."she said sadly.
"E nelle guanc elette
Baciatelo per me-cortesi aurette"
"You used past tense. Isn't it give you hope now?"Sherlock asked, softly.
She was utterly different. Her mind, her eyes, she herself... Was just different. He could manage to see her soul while she lost herself in her poem. A damaged soul.
"Al mio ben che riposa"
A lonely soul.
"No it isn't she said. There is nothing on earth that can give you hope after you lost everything you had."she said.
"Su l'ali della quiete"
A broken heart.
"Grati sogni assistete"
And he saw a little girl who had gotten lost her path in the darkness.
"E il mio racchiuso ardore"
The song was getting more beautiful. And yet, painful.

But instead of, finding the light, she had chose to rule the darkness. But it costed her her freedom. He could see the pure music in her soul. He thought the moment when they danced. She was floating like an angel. And now, no more could he see the shrewdness and the insolence in her eyes. The things he could see was completely different.
Desperation. Innocence. Pain of loss. And she was begging. Something in her eyes was begging. God knows, what for.
He was shocked to see how a music piece suddenly changed her. Her lips were moving with the lyrics of the barouqe aria. Ah, how beautiful it was. She was like the aria. The only way to understand the desperation and the sadness in this music was, listening it with your heart.
He felt some kind of connection to her. To her eyes, to her mind, to her heart. He wanted to put her in a jail, he wanted to make her pay for deceiving him and showing him like a fool in front of everyone however...
He didn't even realize that they got too close to each other.
"Svelategli per me-"
And there was a moment when he didn't see it coming.
"larve d'amore"
It was when their lips had met...

Here is the aria's name "intorno all'idol mio" and it's from the opera, ORONTEA. You should listen Cecilia Bartoli's vertion. She is the best!

Translation of the aria:
"Around my idol blow, just blow
Breezes sweet and gracious breezes
And on the favoured cheeks
Kiss him for me
Courtly, courtly breezes.

My love who rests
On the wings of quiet
Pleasent, pleasent dreams assist.
And my hidden ardour
Reveal him for me
O spirits, spirits of love...