The structure of the Window to the Past is relatively simple; the caster focuses on an object - termed the 'anchor' - around which the spell builds a ghost image, showing the history of the object. If this anchor is say, a brick in the wall, then the ghost image will trace back the history of that individual brick, right back to the time it was fired from clay. This also means that if something accidentally gets in the way, such as a fly, then the whole spell is derailed, showing the history of the fly rather than the intended target. The sensitivity of the anchor-finding part of the spell depends on the talent of the caster. A mage of Twilight's calibre would be able to safely ignore most airborne debris, and maybe even target the contents of packages without opening them; unicorns without much magical training might be able to attempt the spell, only to anchor to the air itself. Maintaining the image is not difficult, but rewinding it is. The further back the caster attempts to see, the more physically draining the spell is, and the fuzzier the image becomes.
Twilight had chosen her position, based on careful study of the story's account, to minimise the size of the image she needed to create. Tail flat against the altar, front and centre. Everypony else stayed well clear, just watching as the faint lilac glow of Twilight's ethereal grip spread along the ground from where she stood to a radius of about five lengths, and up the same distance in the air, forming a hazy bubble. The surface of it flashed white for an instant, and from the outside it looked like the whole thing had disappeared. Twilight remained still and contemplative.
"Did... did it work? I don't see anythin'..." Applejack craned her neck forward trying to pick out the membrane that was there a moment ago. Rainbow Dash slumped to her haunches.
"Pfft... it's a dud."
"Oh really?" Twilight interrupted. She stepped to the side, and in her place, there stood what appeared to be a copy of herself, frozen in time at the moment of the flash. "I haven't wound back yet. This is just the ghost image of when I cast the spell."
"Is it... is it safe?" Fluttershy poked her head around from behind Rarity. The flash had startled her into taking cover.
"It's perfectly safe! Like I said, this is just a ghost image. You pass right through it..." Twilight waved a hoof through her own phantom's head. "... see?" At her reassurance, the others reluctantly stepped into the circle. "Alright. This might get a little weird..." She resumed her wide-legged stance and returned into her near-trance, her horn lighting up again. This light appeared to have no effect on the floor, or the ghost copy of Twilight - both components of the ghost image. Slowly, the image began to move. The ghost Twilight resumed a casual stance, turned, and trotted backwards across the hall. Images of Applejack and Pinkie Pie entered the range of the spell, startling their real counterparts. To make things even stranger, the image of Applejack was cut off at the edge of the spell's range. She wasn't keen to see if the spell's cross-section of her torso was anatomically accurate. After just a couple of seconds, with the rate of the ghosts' odd, reverse movement increasing, the familiar figures left the frame. The only indication that the image was still in rewind was the flickering of the light from the torch's flame.
Twilight grimaced and hunkered down. This was going to be a long and gruelling spell - the target was over a thousand years back, and the image was only looking at an hour ago. With a gulp and a little bead of sweat, she pressed on. A slate-blue blur entered the frame, and disappeared with the torch. This accentuated the odd lighting effect caused by phantom images occupying the same space as real objects; the ponies remained lit by the torchlight, standing on pitch black cobblestone. Outside the area of the spell's effect, room remained normally lit. As the rate of rewind accelerated, the light returned and left again at increasingly brief intervals. Fluttershy took cover under Rainbow Dash, hiding her eyes from the rapidly flashing floor. Pinkie Pie glazed over a little, staring at the bizarre effect. This only lasted about half a minute or so; quite suddenly, the flashing stopped, and the floor went dark. The ponies were still lit up by the torch across the hall, creating an odd-looking contrast to the pitch-black floor. The only indication that the rewind was still in progress was Twilight's visible strain: sparks of etheral energy leaped from the tip of her horn every few seconds, and beads of sweat ran down her face. She had carefully calculated the rate and duration of rewind for the spell, but with all her concentration on the spell, she was reliant on her friends to give her a signal to stop. For ten agonising minutes, Twilight struggled, and occasionally grunted or groaned, while everypony else watched the darkness anxiously.
Suddenly, light burst out. Because of the rate of rewind, the light was flashing even more rapidly than before, day and night cycling by at a terrifying pace. The sudden change elicited squeaks and squeals of surprise from everypony, which, in turn, alerted Twilight to the situation. As her focus was drawn from the spell, the rewind ground to a halt, the disorientating flashing slowing over a few seconds to a constant light level. Twilight opened her eyes and looked up, relaxing her stance a little. The stones on the ground were lit by a phantom daylight, the brightness contrasting sharply with the half-light outside the spell's range. It took her a moment to notice that she was panting and dripping sweat, and her horn was steaming gently.
"Twilight, are you okay?" Rainbow ducked her head down to look at her face. Twilight struggled to respond.
"I'm... ahh... I'm fine." She puffed. She looked around at the brightly lit stones. There was nothing else of particular interest present; all this revealed was the fact that the roof was open at this point. "I need to go further." she gibbered, preparing for another burst.
"Ya sure you can handle it?" Applejack tilted her head in concern. Twilight looked up, visibly exhausted, but still no less determined.
"It's not like we have a choice." she snapped, and ducked her head down, her horn's glow reigniting into a full-on shine. Gradually, the floor resumed its cycle of light and darkness, though at a far less dizzying pace; this was reflected by Twilight's noticeably less pained expression. Not a full minute had passed when the bare stone floor changed to marble tiling and scarlet carpet, and multiple white and gold blurs rushed around the room, neatly appearing at one side of the circular area and disappearing at another. Pinkie Pie tried to keep up with one, only to twirl around and fall over. Applejack gave Twilight a shove to pull her from her trance. The blurs disappeared, and the image froze on a night-time picture of the altar in its heyday. The floor was faintly lit with a pattern of colours, like moonlight through a stained glass window, and a candle on the altar itself. Rarity walked around the side of the altar to get a look at some of the papers spread around on it, only to snub her nose on an invisible wall.
"What the..."
"That's..." Twilight panted. "...the wall of dirt. The image... the image hides it, but it's still there." she grimaced, rubbing her head a little. Rarity huffed and let the papers on the altar alone. "This looks like the palace when it was still in use. That means, we need to go forward..." she closed her eyes, horn lighting up again like a fluorescent lamp, and the image began to wind forward, much slower this time. "... slowly... slowly..." Night passed to day, and to night again, each cycle lasting a full twenty seconds. The blurs returned, still zipping around madly. From time to time, one would stand behind the altar for a brief moment, before dashing off again. It was still too blurred to discern any details about it, beyond its approximate shape and its white colour. About six minutes of cycling later, the soft colours that lit up the floor each daytime were replaced with plain daylight. Rarity appeared to be the only one who noticed this detail; it changed in the blink of an eye. Two more minutes passed, and the texture of the red carpet changed from silky smooth to roughshod, again, rather suddenly. This was enough to prompt Rarity to stir Twilight this time. She was looking progressively worse with each wind.
"Nearly there, darling." Rarity wore the most earnest smile she could, and pointed a hoof at a large impact in the floor, where the carpet was ripped and the tile split, revealing the cobblestone below. "Maybe wind it back a little?" After a moment's hesitation, Twilight wheezed and nodded, and resumed her exertion. The shadows in the image began to move, the traces of clouds racing along, while the shadow of the altar crept along at a snail's pace. Before long - maybe a minute and a half - a new, less natural light appeared, as if coming from the middle of the hall. Over the next few seconds, the light intensified, licks of orange and pink visible on the floor, culminating in a sharp, white flash, enough to startle Twilight, freezing the image just afterwards. With her head pounding and her legs shaking, she struggled to keep the image clear. Applejack put a comforting hoof over her shoulders, being careful not to put too much weight on her.
"Yer doin' great, sugarcube! Now just hang in there..." The earth pony's concerned look was met with a weak nod, Twilight's tortured body straining to stay upright.
This image was definitely not like the others. Tall shadows punctuated the light on the floor, and when followed to their origin, revealed tall figures. The invisible dirt wall made them difficult to inspect up close; they all stood neatly about a foot behind it. However, details weren't quite as important as the bigger picture - everypony (bar Twilight) boggled at the five silhouettes before them in almost reverent silence. "Well..." Applejack gulped. "... I guess that seals the deal, don't it?" Reluctantly, the five ponies approached the dirt wall, getting as close as they could to the shadow casters as they could. The figures stood just over a length tall, two legs supporting their bodies, and two other leg-like limbs sprouting from the sides of their shoulders - to a regular pony, it looked dislocated. A few features betrayed their forms as equine - the shapes of ears, of faces, the mane and tail, the way the bottom legs ended in hooves - but many things just seemed weird about their proportions. The heads looked about right, even if their necks seemed a bit short, but the bodies were much more slender in form, like a normal pony's torso flattened a little. The bottom legs seemed to sit naturally at this weird, vertical angle, and were significantly longer than it looked like they needed to be. And most of all, at the ends of the top legs were these bizarre body structures: flat planes of flesh with what looked like five small legs sticking out of each. They appeared to be prehensile, too; three of the figures were holding stick-like objects with them - an axe, a staff and a spear. Bony spiders...
Rainbow Dash fluttered up to one of the figures. It was airborne, just behind the rest, holding a spear in its... hand, that's what those things are called. She landed for a moment and drew the spear Gilda had given her from its sling, and, carefully balancing it in her front hooves, she held it up to the ghost image for comparison. Apart from the signs of wear and tear on the real one, it was identical. Her eyes traced along the cyan arm, past the tremendous pair of wings, to the face. She'd recognise those mauve eyes, that multicoloured mane, that determined, but subtly playful expression anywhere.
It took Fluttershy a moment or two to come out from hiding behind the altar and take a closer look. Her figure of interest appeared to be landing, its magnificent pale yellow wings spread wide. Everything about her seemed to flow - the mane and tail were at least a length long each, and swept around her form, blending into the soft green robe that billowed behind her. The robe appeared to be adorned with the image of a pink and blue butterfly, stitched on at the bottom. Clutched in one hand was a knobbly wooden staff, mostly unadorned, being used as a third leg on which to hold balance as she landed.
Applejack approached the next one. One feature was calling out to her; she propped herself up on the dirt wall to get as close a look as she could to the hat sitting on her head. Wide brim, tall centre, small indent in the front of the brim. She could have sworn this pony was the previous owner of her own hat, but for one other detail - a small metal ornament in the shape of an apple, clipped to the middle of the hat. That made her smile a little. She wanted one of those.
Rarity couldn't stop squinting at the waist of the one just immediately below the one Rainbow was inspecting. There were a number of details she might have chosen to examine, but she ignored the violet mane curls and shimmering white horn to look at her belt ornament. Three blue sapphires, diagonally shaped, studded side-by-side in white enamel. It stood out so starkly from the plain black clothing she wore; but for some reason, she had chosen to display this familiar icon despite her disguise.
Pinkie Pie gazed curiously at the last figure. Two things held most of her attention: the figure's apron, and the large firework it held atop its shoulder. That thing looked insane. She must be planning one heck of a party! She noticed that both features were adorned with the same icon - a balloon with a string. This symbol seemed to repeat around the nose cone of the rocket, alternating in colour between white and a pale blue.
As Applejack was stepping back from the wall, around the side of the altar, her legs passed through something she hadn't noticed before. A sixth figure was hunched over in the shadows in front of it. A sword - one very like the one Twilight had pulled from the rubble earlier - lay discarded on the ground. The shadow made discerning details tricky, but the light caught a few features from the top of its head that were unmistakable; the tip of a lilac horn protruding from a blanket of straight indigo hair, streaked by the horn with magenta and purple. Following the figure's line of sight, she found a seventh, right at the edge of the spell's range. Only the hooves, legs and white tail tip of this one were visible; the upper portion of its body was outside the bubble. While they reeled from their observations, an eighth figure entered the scene, unnoticed. This one was not like the others - it walked on all fours. More importantly, it walked - it wasn't part of the image. Before they had time to register more than a vague, blue shape, a white flash stunned them.
"Twilight..." it spoke. The unicorn jumped, startled by the unexpected voice. She looked around, still hurting from the spell; though her concentration was now broken, the image remained. Her friends stood perfectly still, some of them in improbable positions - Rainbow Dash floated in mid-air with her wings flared out, and Pinkie Pie teetered on one hoof. Everything was whited out slightly, adding to the parade of unusual lighting effects. Everything, except for the pony that now stood in front of her. Slate blue coat, pastel blue mane separated by a horn, a midnight blue ornament around her neck, adorned with a white crescent, and two impressive wings tucked into each flank. She couldn't quite believe her eyes; she blinked incredulously, not sure if this was her, or just her tired mind playing tricks on her.
"... Luna?"