The day was peaceful.
It never lasts.
The first harbinger was a low thrumming sound that hummed through the air. It was noticeable, but not unduly so; it was the kind of thing that made ponies look up in the air for a moment in confusion then return to their business. Only four ponies in the entirety of Equestria noticed anything at all: the Tetrarchy. First among them was Celestia. She was attending her day-to-day court business as she always did when the sound began. Immediately her ears fell flat against her head and her eyes widened. Observers would later comment on how her mouth had moved as though she was saying something, but none of them actually heard it before her horn coruscated with golden light and she was gone.
The second was Twilight Sparkle. She was walking home from a day at Whitetail Woods with her friends when the low sound made itself heard. She started in shock before crying out in panic and vanishing in much the same way as Celestia, leaving her friends extremely confused and with no idea where she'd gone.
The third was Cadance, who was in a rather similar state as Celestia: managing court affairs in the Crystal Empire. Instead of immediately teleporting, she began to heave enormous sobs and ran out of the court and into her private chambers before explosively disappearing just like her two fellow princesses.
Luna had spent all night watching over the sky and over the dreams of their subjects, and so she was asleep.
And then she wasn't, because Celestia had appeared in her room and grasped her by the neck before teleporting them both away once again.
The four of them convened in the Upper Halls of Canterlot Castle, where they normally met only once yearly to discuss what had happened during that year. Thus meeting there out of time was quite out of the ordinary. All four were there: Celestia, her face set in a stony silence; Twilight, eyes wide with shock and alarm; Cadance, red-faced and still crying; and Luna, confused and barely-awake, but still registering the omnipresent sound.
"Twilight," said Celestia, "I need you to get Discord here."
Before Twilight could even respond, the draconequus in question appeared above the round table that they were gathered around. He was as they'd never seen him: despondent. Everything about him spoke of complete, profound sadness. He had no jokes or wisecracks. No matter what was asked of him, he simply stared morosely at the four around him before snapping his fingers and disappearing only thirty or so seconds later.
"W—what's happening, Princess? What's going on? I feel like something's taking my magic apart!"
Celestia sighed. "What's happening, Twilight, is..." she swallowed heavily, glancing at the other two princesses in the room, "...the world is coming to an end."
At the expression on Twilight's face, she continued. "You can't stop it. I can't stop it. I entertained a hope that maybe Discord could, but that possibility is exhausted now as well. I'd say that at the moment we all have maybe ten hours to live. I'm going to go to the source of the disturbance to try to extend the time window, but within the next month at the most, everything is going to end." Before Twilight could say another word, Celestia teleported away once again.
"Wait, Aunt!" cried Cadance, who teleported directly afterwards.
Only Luna and Twilight were left.
"...Luna, w—what's happening...?" stuttered Twilight.
Luna grimaced. "Twilight, do you remember when you traveled a week back in time to try and stop yourself from doing exactly that?"
The smaller princess slowly, mutely nodded.
"Well, Twilight," continued Luna, "when you did that, you stitched together two previously-unconnected timelines and unknowingly accelerated a process that normally occurs once every few epochs. From the few ruined texts that Sister and I have found from the distant past, we call it a Ripple. The threads of time—every timeline, every universe, every parallel—are constantly moving, and once in a very great while, they slowly draw from a loose skein into a tightly-woven twine. All of time, all of all times, draws together and the world ruptures for a brief moment. Chronic energy cracks the Central Leyline that runs down the center of Equus and carries all of the magical energy in the world. Tell me, Twilight, because it seems to me that you would know of this: what would happen, should the Leyline crack?"
Twilight's eyes widened ever further. "It would unleash an outpour of raw magic onto the world. That much energy would be enough to..." she trailed off, her face twisted into a gape of horror. "...overload the magical circuit of everything on Equus and..." With a cry of despair, she teleported away, leaving Luna alone, looking up to the castle's marble roof.
"Does it really all come to this...?" she mumbled, "all of the ponies' work, all just to be destroyed at the whim of the timestream?" Her eyes narrowed. "I refuse this."
She flapped her wings and darted at the window, phasing through it as though she was nothing more than a phantom. The humming changed pitch. Rotating in the sky, she turned to where it was the loudest, towards the setting sun in the west, where the Central Leyline lay. Laying before her, beyond the horizon, was Eternity's Leap, Vaporis, Abaddon's Mouth and many unexplored lands besides...So many miles that she'd not seen in many, many years. She dared not teleport. It had been too long, the world had changed too much. She would travel there by wings alone to the Old World. Celestia had raised the moon for a thousand years. She could manage a month more.
On silent wings, she flew west, chasing the sun towards the ever-expanding hum of the Ripple.
It changed pitch again.
Behind her the moon slid over the horizon, drawn up for the last time in many days, she was sure, by her blue-indigo magic. The dying light ahead of her flew faster than she ever could, and in moments, the world fled into the quiet dark of night, Her wings hurled themselves forwards with greater resolve. The last light of the sun reflected from her eyes before it faded entirely and she entered her element. The world was black.
The Ripple hummed on.