A small change results in much bigger difference. Twilight, sent to alternate reality in "The Cutie Re-Mark", realizes that in order to undo Starlight's spell, she needs the help of one who created it.
Searching for her, Twilight finds both best and worst versions of not only Starlight, but herself, in many different realities. Forced to see the extremes of both of them, she is forced to ask herself – should she try to save Starlight, instead of fighting her?
But the real question is – can she?
"I'm telling you, Spike. I think I saw Starlight there." Twilight sounded worried as she trotted through Ponyville, her saddles full of equipment and notes she used during her presentation. "She was sitting in the audience for a moment, and then disappeared. I think I saw some shimmering, like she went invisible."
"What is this villain doing here?" Spike wandered, even more worried. "She must be here for revenge, for sure."
"Possibly. But why would she go to my presentation?" Twilight wondered. "She is much, much more talented and has much more knowledge when it comes to cutie marks then me. Yes, Spike," She interrupted her assistant before he managed to protest, "I may be the Element of Magic and an alicorn, but cutie marks are literally what she specializes in. Remember when she stole our cutie marks when me and the girls when to her village? I still cannot replicate that spell, to be honest. I know how to defend against it, but that's as far as I went."
"That's… worrying. Think she is going to try and steal your cutie marks again?" Spike asked, opening the door to the Twilight's crystal castle. "Should we warn the girls?"
"I think it's going to be something else. She already tried that and failed, and I doubt she's going to just do the same thing again. But yes, we should, I'm going to go and make sure they're ready for anything." She got lost in thought for a moment. "I think it may have something to do with Cutie Mark Crusaders. They were the only truly interesting part of the story, with them acquiring their Cutie Marks simultaneously. We should warn them, as well."
"Yeah, I don't want them to become victims of some sadistic experiments." Spike shuddered, walking to the map room. "Who knows what this mare is capable of. She's totally crazy."
"Exactly." Twilight entered her throne room, with the inactive Cutie Map in the middle of it, still lost in thought. "I can teach Rarity and Sweetie Belle a counterspell to defend themselves, but…"
"Twilight!"
She looked around when she heard Spike's sudden shout, only to see Starlight Glimmer sitting like nothing happened on Twilight's throne, with her legs on the Cutie Map. A scroll, pulsing with magic, was hanging in the air above it, giving off strange blue-white light that Twilight couldn't help but think seemed eerily familiar.
"Hello, Princess." She smiled in a sadistic, evil way that made Twilight shudder. "So nice to see you. Shame it's for the last time, but at least I got to see your pathetic face before I defeat you."
"What?" Twilight fired a destructive spell on the scroll, not wanting to see what the crazily evil mare had in store. It deflected off the magical shield, ricocheting into the wall. "What are you doing, Starlight?!"
"Your primitive mind cannot even comprehend this magic, can it?" Concentric rings of blue-white light appeared, surrounding the table and the air above it. Twilight fired another spell, this time a stun one, at Starlight only for it to be blocked by the weird rings.
Wait. It wasn't blocked in a normal way. It... oh no. No no no!
"Starlight, listen to me. I made this mistake before." She tried to reason with her enemy, as the air grew charged with power she knew all too well. "Time is unchanging. If you go to the past, you will only reinforce what already happened!" She really hoped she was right. She couldn't imagine what someone as evil as Starlight could with with actual, working time magic.
"Oh, that's what you think. I heard you finished Star Swirl's the Bearded spell to get those wings, didn't you?" Starlight started to levitate, as a black hole appeared in the ceiling. It wasn't a natural, black color; it was an absence of everything. Perfect nothingness… at least when looked at from this moment in time. "Let me tell you something, Princess… I did it as well. Just a more... useful spell, it seems."
And with this, Starlight disappeared. The spell triggered, and suddenly, everything started to shimmer and glow. The palace, Twilight, Spike, everything.
"Twilight! What's going on?! I feel… weird…" Spike tried to hold the wall, only for it to start disintegrating into pieces of multi-colored light.
Twilight tried to cast a time-stabilizing spell, but it instantly failed. The reality, the timeline they lived in, it was starting to collapse. Somehow, Starlight did the impossible. She destroyed the entire timeline. Twilight started to panic, thinking of any way to stop it. But there was none. They were going to disappear. This timeline, she, Spike, all her friends, even Celestia would literally never exist. As if they never did.
"Twilight, the scroll!" Spike didn't have much knowledge about magic, but his years of experience with Twilight taught him enough to recognize basics of magical emergencies and what to do. And the scroll with whatever spell Glimmer used could only help. He grabbed the scroll – which, somehow, seemed to be the only thing not disintegrating – and showed it to Twlight. Or at least tried; instead, he tripped on something, a literal hole in reality that opened under his feet, and fell on Twilight, the scroll touching her sides.
They were both suddenly enveloped in white-blue lighting, and moments later, a black hole opened above them. Both screamed with surprise, as they were suddenly pulled into it, disappearing.
Everywhere else in Equestria, the world was ending forever.
Luna woke up from her sleep, and instantly teleported to Celestia, hugging her sister in their last moments. Tears flew openly, as they both cried, some scared guards joining them in a hug as they all felt pieces of themselves disappearing.
On a farm near Ponyville, Apples huddled together. Applejack tried to keep the brave face, trying to make sure Apple Bloom won't see her tears.
In Ponyville, Rarity and Sweetie Bell were also hugging, the little unicorn's last thoughts being of her sister and other Crusaders as all her other thoughts disappeared into nothingness. In the ends, even these small parts of her dissolved into silence.
Scotaloo hugged Rainbow Dash, who was training with her moments before. They both were trying to look brave in front of the other, and even partially succeeding, as darkness encroached upon them.
Fluttershy calmed her animals, herself scared, but not willing to let her little (and big) friends be afraid. She managed to calm them down, and they all hugged her as the end approached. Even Angel. Soon, all perished; only the remarkable stubbornness and loyalty of the small animal remained, for a moment longer, facing the end until it, too, ceased to exist.
Countless magicians tried to put up powerful wards and spells. Chrysalis screamed, her powerful throne giving her no protection from the coming end. In Tartarus, the most powerful of villians shouted and screamed in hatred and despair.
Discord had no idea what was coming before it happened. It was so rare to him to be surprised, in his entire life; it barely ever happened. The Lord of Chaos savored the surprise, as he learned what happened and realized that the end of everything is now coming. He sighed. Well, what can you do, some things even Chaos cannot stop.
Get to fixing it, Twilight. I have a tea with Fluttershy soon, and I don't want to wait eternity for it, he thought as the end approached him.
And in Canterlot High, Sunset Shimmer and her friends stood together, in vain trying to use their powers to stop the end. As they failed, and the world fell apart, they stood together until the end, with only human Pinkie Pie still smiling; but somehow, her unending lack of fear made them all smile weakly in their last moment, banishing all but the feeling of friendship.
Back in Equestria, Pinkie Pie looked at the coming end. She wasn't scared; there was nothing she could do, and she knew it. Why fear something you cannot do anything about? So she just closed her eyes, and said her last words without any doubt or hesitation, and with a smile on her face.
"Everything's in your hoof now, Twilight. I believe in you!"
And with these words, the bearer of Laughter surrendered to nothingness.
The title of this story and the first chapter is a reference to a great MLP fan song, Resilience, available on PrinceWhateverer channel (not a typo). My favorite fan-made MLP song for sure, I highly recommend checking it out.