Slightly before it all...
Twilight and her friends had left the city several hours ago, at dawn, following Celestia. Now, with the sun already halfway up the sky, they saw for the first time a sign of something built in the strange wilderness below: eight ornate marble columns, surrounding a circular basin of the same stone, open to the sky. Just ahead of them, Celestia began her descent, apparently aiming for a narrow patch of cleared flagstones just on the basin's lip. Twilight looked over at Applejack, shrugged, and followed.
As the other six ponies approached, they saw the flagstone was not bare: a small statue, half-overgrown, sat on the edge of the basin. Celestia was already clearing it off with her magic, carefully sending the vines and roots into the thick growth around them; the basin itself was curiously spotless. She finished as they walked beside her, and saw the figure was that of an alicorn, smaller than any they'd seen, and curled tightly around herself as if asleep, but unmistakably horned and winged.
"This place has fallen on harder times than I'd hoped," Celestia said, seemingly to herself, "but the pool itself is clear. It should still work," she laughed here, high and clear, "though, for all I know, nothing could stop it working."
"That's great, Pr-err, sugarcube," Applejack said, "but what in tarnation does it do when it works?"
Celestia turned to her and smiled, and lit her horn. The status lifted its head, and opened its small mouth. Twilight did a double take, she moved so smoothly, and had to look again and see the telltale marble sheen to be sure she was really stone, not a living pony. Before Twilight could register her surprise, though, a healthy flow of water began spilling from the statue's mouth into the basin, and all of them immediately turned to look.
The water was breathtaking, mesmerizing, shimmering in the late-morning sun in an array of lights and darks that Twilight at first thought must be the result of a perfect angle of the light. Then she looked closer, and was no longer sure it was water at all. The liquid spilling forth looked similar to Luna's mane: dark as night, filled with stars. She looked to where it was beginning - faster than it should be - to fill the large basin, and it looked like nothing so much as the night sky itself was seeping into the pool, stars swirling and churning as if a thousand nights were passing at incredible speed.
Before she'd really had time to finish gaping, it seemed to finish. The alicorn statue's mouth closed, and it curled up again, and the strangest sight now greeted them: the basin seemed to be filled with a pool from a different place and time, reflecting, rather than the bright daytime sky above, an utterly clear star-studded night.
"Oh my," Rarity breathed. "Those aren't our stars. What is this...Celestia?"
"The next adventure," Celestia replied, "should any of you wish it. You've of course already done more than enough, but lately I get the feeling some of you have grown restless," here Rainbow Dash nodded, and Pinkie's perpetual smile faded slightly, "and I realized that here, I have adventure enough even for those as experienced as you. This, Rarity, is a map, and, with the right spells, also a gateway."
"A map of the stars? But Rarity's right, this doesn't look like our night sky," Twilight said.
"Not quite a map of stars, though I think perhaps, soon, it will not be so far off. But no, this is a map of worlds."
"Ooh, like that weird human mirror world?" Pinkie asked.
"Nor quite like those, though closer. Magic, as you know, is full of surprises, and the mirror worlds are a part of it even I do not understand. They are part of Equestria, and depend upon it, as you must have noticed. I wonder, sometimes, if the mirror creates them to reveal things about us, or show us how things might have been. But you could not travel to them without the mirror's magic. They do not exist in that sense. These do, or may."
"So these are Planets then? Alien planets?" Twilight nearly bounced with glee.
"Indeed," Celestia smiled at her, "though not as alien as you might think. If you go to one of these worlds, you will likely see a great deal that is familiar. Even before Luna and I arrived on Equestria, all these worlds were one vast land of wonder, so large no pony, not even I or my sister, could possibly travel it all. Then the story changed. Now they are these many worlds, scattered through the sky.
"When it all changed, it happened so thoroughly that now by all physical indication, they have always been separate, since the dawn of time, save for curious similarities. Some have ponies, or creatures very like them. Some," she smiled at Applejack, "have apple trees. Stories of creatures that now exist only on other worlds persist elsewhere, memories of times when one could walk to visit them. Now, if you are willing, you could visit them."
"Is that it, then?" Rainbow leaned back and cocked her head, looking skeptical, "just go visit some other worlds for fun? Don't get me wrong, it sounds better than sitting around in the city all day, but I thought we were talking adventure. Something important. Fate of the world stuff. I liked saving the world."
Celestia frowned a little. "Do not misunderstand, Rainbow. These worlds are not safe. Many need saving, or are more complicated. None are identical to Equestria, and some are defined as much by what they lack as by what they share. The Tree of Harmony was unique in all the world, even before it changed. Friendship carries great power everywhere, but not always of the same sort, or so direct. Some, until recently, lacked magic entirely, and so have no creatures that can sense or use it. And some are made of lands that were always frightening to us, full of creatures with very different goals and views than ponies. The less frightening ones are powerful beasts that only seek to consume and destroy. The more frightening, well, even for you, perhaps we should not yet speak of them. We would need to choose which worlds you visited first carefully.
"But do not misunderstand. This work is important. The physical world may not remember that it was once all one, but magic does, and ever since the world split, it has been working, as much as possible, to bring the worlds back together. That time is growing close. Magic has crept back everywhere, the whole night sky, and it will not be too long now before others learn to harness it, to ride the currents of magic and move through the void much faster than they previously thought they could. Then all of us will begin to meet again, and a new age will come, for good and ill. You could be among the first to visit these new places, to form their opinions of ponies, to bring back some harmony they may have lacked. To prepare them to work together, against the many that will prefer a different path."
"Um, you mean like when we told everypony about the magic of friendship?" Fluttershy asked.
"Yes, but instead of Equestria, you would be spreading it throughout other worlds."
"That sounds...nice. Scary, but nice."
"I am so in," Rainbow Dash nodded.
Slowly or quickly, the rest agreed.
"I am so proud of you, my little ponies," Celestia said, "proud, but not surprised. In truth, I expected as much, and have several possibilities picked out. We should go back home for now, and look at a few candidates before we decide. It will still be nearly a century, I think, before the first worlds start to meet. Perhaps a bit less, if we decide to push it along. But I have also never used this pool as a gate before. While I am quite sure it is safe, the changes in magic may have unexpected results, or adapt you to the local world in strange ways, at least a first, in the same way the mirror turned Twilight into a human. I will do what I can from here, but we will need to choose somewhere reasonably safe for our first test, though of course we will pick somewhere that looks like it could use your help. I suggest somewhere without native magic users, so your magic will be better able to keep you safe, but we can sort it out. I admit, despite not going myself, I'm excited!"
Pinkie bounced happily at this. "So am I! There are so many friends I never knew I had!"
They all smiled at her, and got ready to head back, eager to decide where to go first.