vworrrrrrrrp... vworrrrrrrrrp...vworrrrrrrrrrp
The door to the TARDIS opened, and Twilight and the Doctor cautiously peered out. They both glanced first at the moon; pristine and white.
“Well, Luna hasn't been banished yet,” said Twilight. “We seem to be right on time.”
“Of course,” said the Doctor. “Now, how can Paradox prevent Nightmare Moon's defeat?”
“The easiest way is probably to somehow sabotage the Elements of Harmony,” said Twilight. “Celestia was reluctant to fight against her sister, while Nightmare Moon was more than willing to injure Celestia; without the Elements, I'm not sure Celestia could have made it.”
“That makes sense,” agreed the Doctor. “Do you know where the Elements are in this time?”
“Under the Castle of the Two Sisters,” Twilight said, promptly. She pointed at a nearby building.
The Doctor nodded. “Then let's go check up on them,” he said. “Does it have tapestries?”
“Yes,” said Twilight. “It's famous for them.”
“Good,” said the Doctor. He cantered along the side of the building, until he reached a door. He knocked, and the door instantly opened.
“Tapestry inspector,” said the Doctor, not paying attention to the pony who had opened the door, “come to inspect your – oh.”
Paradox grinned at the Doctor. He was wearing a bulging pair of saddlebags.
Twlight, who had been paying attention when Paradox had opened the door, had her wings spread slightly, her head lowered, and her magic ready.
“Well, hello, Doctor,” said Paradox. “I must say, I really thought I'd lost you with that last jump.” He stepped outside the palace, closing the door behind him. “Not that it really matters,” he continued. “You're not going to exist for long. Oh, and do tell your pet alicorn to stand down, would you? There's nothing either of you can do now to save yourselves.”
“I doubt that,” said the Doctor. “I doubt that very much indeed. Let me guess – you've sabotaged the Elements in some way?”
Paradox smirked. “I've stolen Laughter,” he said. “Without one of the Elements, their power is weakened; Celestia will die, and by the time you arrive, Doctor, injured and alone, your TARDIS weakened and damaged, there won't be anypony left alive. Without a safe place to rest and recover, you will die, Doctor. You will die, and then I... I will never have existed.”
Twilight frowned. “You... you want to not exist?” she asked.
Paradox's expression darkened. “Do you know how I escaped from Discord?” he asked. “He never took away my time machine, no. He just activated it, sending me back to the beginning, time and time again. There was no escape through time, Princess. So I did the only thing I could.” Paradox strode forward, forcing Twilight back. “I waited.” said Paradox. “I was his plaything, Princess. I was the toy that never broke. But just because I can't be injured doesn't mean I can't feel pain, Princess. I had to wait him out, Princess, destroy my time machine and rebuild it after his defeat. I had to wait, I don't know how long, it must have been at least three hundred years. And not just once, Princess. Time and time again. And every time, there's only one thing that keeps me sane, Princess. Knowing that when I get out, I am going to make it not have happened. The first thing I do when I get out, Princess, every time – I try to kill you. And this time – I'm going to succeed.”
“No, you're not,” said the Doctor. “I'm... sorry. I really am. But you can't win here. It's temporally impossible. You'd know that... if you were honest with yourself.”
“I'LL KILL YOU!” yelled Paradox, leaping for the Doctor. Before he could reach his target, though, Twilight grabbed him in her magic, and lifted him up and away.
“So we just need to take Laughter out of his saddlebags, and put it back with the other Elements?” asked Twilight. “This should be easy.” The left saddlebag opened, and Twilight pulled out a familiar blue crystal.
...a whole series of familiar, and identical, blue crystals.
Paradox laughed maniacally. “It doesn't matter!” he cried, triumphantly. “There's no way you can tell the real from the fakes in time! Even if you manage to get one in place, it'll never be the right one!”
“Telling a real Element of Harmony from an inert sapphire is a fairly simple spell,” said Twilight. “It only takes a minute or so to cast.”
“And just how many minutes do you think you have?” snarled Paradox.
* * *
“There can only be one Princess in Equestria! And that Princess,” Luna said, stomping her forelegs, and destroying the bridge that symbolically connected her and Celestia's thrones, “will be ME!”
* * *
Twilight looked up as a window in the Castle of the Two Sisters suddenly blew inward. And then she looked further up, as the moon moved directly in front of the sun, and an unnatural night fell over the city.
“...not enough minutes,” she admitted.
A triumphant burst of evil laughter sounded from inside the Castle.
“Look, Paradox,” said the Doctor, “it's still possible to help you. We don't have to fight. You don't have to kill anypony.”
Paradox grinned. “Too late, Doctor,” he said. “I've killed you all already. And if it doesn't stick then I'll do it AGAIN and AGAIN until you're DEAD!”
Twilight shook her head. “No,” she said. “I think I know how to fix this.” She glanced up, as a beam of cyan light sliced out through the roof of the Castle.
Paradox screamed incoherently, legs flailing as he struggled against Twilight's magic. “I'LL KILL YOU!” he yelled. “I'LL KILL YOU I'LLKILLYOUI'LLKILLYOUI'LLKILLYOU! I'LL KILL YOUR PARENTS BEFORE YOU WERE BORN! YOUR GRANDPARENTS! GREAT-GRANDPARENTS! I'LL GO UP AND UP THE FAMILY TREE UNTIL I SUCCEED!”
Twilight calmly put the crystals – all of them – back into Paradox's backpack. And then she pressed one hoof down on the bulge at the bottom of the flower that Discord had given her.
A stream of strawberry milk spat out at Paradox, and immediately grabbed his time machine. The milk hit several buttons, and with an even louder scream of rage, Paradox vanished.
One single blue crystal remained where he had been. From its position, Twilight guessed that Paradox had swallowed it.
“The Elements of Harmony,” said the Doctor, recalling their earlier conversation with Discord. “Can't travel through time.”
“Exactly,” nodded Twilight. “I've got to -”
“Put that back and then get out here so I can yell at you!” said the Doctor. “Do you have any conception of what you just did?”