The horizon was temporally isolated from the rest of Equestria—time had little meaning here, and it advanced only as quickly as the those within it perceived it to.
"Where are you, Dia?" Celestia sang, from a time four centuries past. Her pink mane swished back and forth as she prowled, though the golden tiara perched atop her head would keep it from becoming disastrously unkempt. The deep arbors transformed to dank, sopping grottos in the mind's eye of the royal filly, and the pair's game of hide-and-seek an anxious contest between predator and prey. "Come out, come out, wherever you are..."
Obscured from the young pony's view by the trunks of the trees surrounding her, the two adult princesses watched her with profound curiosity. Celestia found that her mouth was unhinged and hanging open, as if the shock ignited within her by this specter of the past had required a route of escape. Circadia, contrarily, seemed something less than surprised, and even quipped: "Oh, I remember this! I'm about to jump out from behind that raspberry bush and surprise you. I was quite the devious little filly."
Her composure failing her, Celestia pounded a hoof into the verdant lawn in frustration. "This can't be possible, Circadia," she declared exasperatedly. "I don't even have my cutie mark—how can we be seeing something that occurred nearly four hundred years ago?"
"I don't pretend to be an expert regarding the mysteries of the horizon, Celestia," her cousin admitted, "but it would stand to reason that, if time has no real meaning here, that our past, present, and future would unite into a single entity."
"Still, if we remain here any longer, she'll surely—I'll surely notice us, and I certainly don't remember ever meeting my future self while playing hide-and-seek," Celestia said. Her past self paused, glancing first to her left and then—as the older princess held her breath—to her right. But the filly did not notice them, nor gave any indication of such.
With a roar, the young Circadia launched herself from her hiding place behind the raspberry bush and tackled the unsuspecting Celestia into a small oak sapling. "Sneak attack!" she hollered. The infant tree bent beneath the weight of the two princesses, finally snapping and sending the both of them tumbling to the ground.
"Well, perhaps they exist on a separate plane of existence," the elder Circadia conjectured, though her tone suggested a lack of confidence in the idea. "Our ability to see them is merely an echo of our memories. A vivid illusion, nothing more."
"You're standing on my mane!" whined Celestia, the filly held prone against the ground beneath her cousin. "Get off! Get off!"]
"Sorry, Tia," the filly with the green mane replied, rolling to one side and allowing the heir to the Equestrian throne to rise gingerly to her hooves. "Didn't mean to hurt you like that. Besides, I can beat you even without the element of surprise! En garde!"
The fillies tussled, still wholly unaware of their older selves observing them from upon a grassy knoll at the center of the clearing. Slowly, a grin stretched itself across Celestia's face. "You could be right, Circadia," she allowed. "We'll wait, and we'll see what happens. Until then..." She trailed off. Her grin faded.
Understandingly, Circadia finished the thought for her cousin. "...you needed my counsel regarding your sister." She laid a comforting hoof on Celestia's shoulder. "Of course. We won't delay any longer. We'll find a peaceful place to lie down, and I'll have the doves bring us some ambrosia."
The mention of that most heavenly of all nectars—a product of the great ash tree at the center of all creation, one that Celestia had not tasted of for more than one hundred and fifty years—brought a certain warmth of anticipation to the princess's heart, though it was not enough to banish the cold pit which Luna's betrayal had dug into that organ. "I would like that," Celestia said, with a small nod.
It's been long enough that I figured I should post this first part and add the next once I'm done editing it.