Phantom

This has to be a palace, or at least a governmental office of some kind, Aria concluded.  The great lengths taken to pretty the place up were the key.  The only establishments that went to such lengths for decorations were theatres, expensive hotels, and the aforementioned royal lodgings, and she could rule out the first two if only for the fact that there was a secured vault on the premises that the Empress was confident enough in to hold her treasures.  It made no sense for such a thing to be anywhere else.

Moreover, the pony Empress (and for that matter, everypony else) had seemed uninterested in her.  They either didn't care that she was moving about...or they couldn't perceive her, for whatever reason.  The former possibility was odd, but the latter was interesting.

Aria sidled behind a column and sat down, just to be on the safe side, as she pondered.  Review the facts again, she told herself.  Vault, probable castle, Empress, potential invisibility.  The first two could be confirmed by a quick look around, she reasoned.  The third was more a statement of fact than anything, and the last...that would need testing.  She leaned out from behind the pillar, awkwardly holding on with one hoof and wondering how ponies demonstrated the level of dexterity she remembered seeing in them.

The hall remained deserted...but it turned left at one end, and she was reasonably sure she heard voices that way.  She scuttled down the carpet, sparing glances at the paintings and décor around her as she went.  She was surprised at the paintings on the walls; many of them recounted scenes she'd never heard of before or fables she'd never read.  One vase on a pedestal to her right was dimly familiar; she was reasonably sure she'd seen ones like it during her time in Canterlot so far back.

She smiled wryly as she crept along; it hadn't been that long ago, by her reckoning.  They'd been torn from Equus and stranded on Earth for months...perhaps a year or more?  Aria couldn't be sure; keeping accurate track of time had been a tertiary concern at best to Adagio, who was indisputably in charge of their 'operations'.  Their illustrious leader (such as she was, Aria snorted) had zeroed completely in on the first task of survival and the second of finding a way back to Equus and Equestria after that, if possible.

Going from place to place, using the scant powers available to them to sow discord and reap the heated emotions therein...and each time, they barely broke even.  Humanity, for all its history of violence she'd read about, had so little emotional energy to take.  It was baffling.

Aria rounded the corner and found to her annoyance that an even greater hallway greeted her.  On the other hoof, this one seemed to be a primary pathway, as it branched off into numerous arches and doors.  She was on the right track, at least...

She paused as a pair of heavily armored sentries marched from a side hall, a certain ease in their movements that told her they weren't expecting trouble.  As much as they were making a show of scanning everything around them, they were lax in their scrutiny.  Aria chuckled under her breath; these were the Empress's palace guard?

Still, they were well armored and probably quite strong for carrying around all that heavy plate for any length of time.  If they were to detect her, she'd have to run and she wasn't confident in her agility in this unfamiliar form.

It was now or never, she supposed.

Gingerly, she stepped out into the hall, the echo of her hooves frightfully loud as she shouted at them,  “HEY, BUCKET BRIGADE!”

The sentries stopped cold; Aria's heart dropped to her stomach, so to speak.  They turned about...and resumed their march back the way they'd come, giving no indication that they'd heard her or that they could see her standing before them.

The siren hesitated as they passed by her.  They really couldn't see her.  That was...good.  Probably.  She could move about freely.  Still, it was troubling.  How had she come to be a virtual ghost of a pony?

Aria shook her head; there was nothing for it but to go forward, after all.  Perhaps Adagio and Sonata were in a similar state and she could find them and they could sort out what to do from there.  She turned about and made for the nearest branching hall.

A few hours fruitless searching found her in a garden, grumbling to herself as she trudged to a halt, taking up a seat on a stone bench as she caught her breath.

Nothing.  No signs of either of the other sirens; she'd even back-tracked to the vault door to be sure.  Admittedly she'd spent a lot of time struggling to remember what an uncountable number of crime-solving stories and shows had taught her, but actually applying it was a lot trickier than it seemed, and even then, she wasn't totally certain what she was supposed to be looking for.  She pulled a  violet hair from her forelimb with her teeth, wincing at the momentary pain as she dropped the now free fuzz to the bench.  It definitely existed, she mused.  If the others were in a predicament like hers, they would've certainly left some kind of sign behind.  A curly goldenrod strand, or a long turquoise one.

Not for the first time in her life had she lamented the relatively plain eyes sirens were born with; she could really use a magnifying glass or something.

Aria was silent for a moment as the absurdity of her last thought overcame her.  She was unable to restrain a laugh as she tried to figure out what she would even DO with a magnifying glass.

“Is somepony there?”

And just like that, she froze mid chuckle, clapping her hooves over her mouth in a panicked effort to stifle her voice.

“...hm.  Mysterious...”

The sound of metal on stone amplified Aria's horror; as far as she could tell none of the ponies in the palace had worn any kind of shoes.  That could only mean that this was the Empress.  She slipped down and under the bench as quietly as she was able.  She couldn't take any chances with that pony; she was visibly too powerful for Aria to do anything about in her current state as some kind of invisible interloper.  The hooves paced around the nearest hedge...

Aria was taken aback to see slender, deep blue legs atop obsidian shoes, a vision of the night sky trailing behind them.  That wasn't possible.  The Empress had been white with gold raiment, and a very different mane.  Who was this, then?  The legs came to a halt before the bench, the tall pony turning in place as though looking for something or someone.  The siren held her breath and remained as still as she could as this different creature continued to cast about after her.

“...perhaps there was naught here after all.”  The mare's vernacular almost made Aria giggle; she hadn't heard anyone speak like that in quite some time.  Either way, the mare turned to leave, her loud hoofsteps allowing the siren to track her position until all was silent.

The siren-turned-pony crept out from beneath the bench, looking both ways and exhaling softly.  She'd have to be more careful; if someone COULD hear her, then someone could catch her and then she'd never find her sisters.  She could see the gentle oranges of twilight starting to form on the horizon; she would have to find a place to rest until the morning; preferably a hidden one.

It wasn't until she made to leave that a glint of light on the bench caught her eye.  There, atop the seat, was an diadem made of the same dark material as the hoofshoes of her mystery royal.  She startled backwards.  This pony was either incredibly naive, or she'd been found out, and she wasn't taking a chance on the latter.  Aria turned to run...

...Only to find that she couldn't move.  A strange blue haze lifted her from her hooves and made moving her legs like trying to tread molasses.  She strained from the effort, a groan of exertion leaving her mouth as she craned her head around.

There behind her stood the blue Empress, a hard expression on her face as a field of stars danced around behind her, carried in a violent ethereal wind with her wings flared.

“And what have we found...?  A thief, a spy, or perhaps a foolhardy youth bent on proving her courage?”  She paced closer, her eyes luminous pearls in her skull as her face shifted to a grim smile.  “We'll soon find out.”