"Just imagine it!" Twilight Sparkle declared, holding up the crackling cylinder. "Ideas, transmuted into Aether! The ability to absorb the knowledge contained within an entire library with a simple spell and a few seconds of one's time! Think of the time it would save, the averted wear and tear on books! This could render the very concept of the lecture hall obsolete!"
Starlight Glimmer took the cylinder from Twilight's magic, and began to examine it in detail. Like most brilliant designs, it seemed deceptively simple; a tube of glass as thick around as a hoof, stoppered on both ends with huge caps of gold, and a purely ornamental gold chain that would presumably be used by non-unicorns to carry it with less risk of breaking the glass. The only remarkable thing about the cylinder was what was held inside; a roiling mass of crackling blue clouds, which seemed to whisper the glossaries of encyclopedias into Starlight's ear if she held the cylinder close.
Then she looked at Twilight Sparkle. In this timeline, she seemed very much the same as she always was before she ascended; she was studious, more than a little bit bad at the whole concept of interpersonal relationships, and surprisingly difficult to tolerate when she wasn't reflexively kowtowing to her mentor. All of this, Starlight had gotten very used to seeing. But there were certain key differences that she just couldn't ignore.
The first and most obvious was that Twilight Sparkle was completely covered in gold. If Starlight had ever thought that Celestia's regalia was gaudy and tacky, she never would again after seeing this version of Twilight. She had golden necklaces, golden anklets, an actually rather fetching sort of golden halo thing, and some very oddly placed gold chains and iron piercings. Her horn even had a thin golden spiral locked around it. If not for her surroundings, Starlight might have thought that Twilight was a queen.
But her surroundings were not those of a queen, or even those of somepony who was rather well-off. It was a strange combination of a library, laboratory and mold-infested crumbling temple. The massive hole in the wall and ceiling was almost bigger than the room it led into, and the world outside, beautiful as it was, looked to be a dense jungle that was miles from any civilization, and the only actual door out of the library had been collapsed from the inside. If Starlight had to guess, she'd have guessed that Twilight Sparkle was a prisoner.
"Are you done staring at the Knowledge Tube?" Twilight's voice cut in. "Only, you've been staring at it for more than a few seconds, but you haven't touched it to your head, so it shouldn't be telling you anything, and I really do need to make sure it works, so could you please..."
The way that last note of Twilight's voice petered off made Starlight's entire body twitch with the urge to smash what was clearly the result of, at the very least, several months' hard work and then blame Twilight for it. Out of the corner of her eye, she noticed that Twilight was already inching towards the laboratory section of her chamber, as though quietly resigned to having to start all over. So instead, Starlight turned around and carefully placed the cylinder back on the table.
"Sorry, Twilight. It's just... such an amazing achievement, and it's so very pretty." Starlight gave her most genuine smile, and Twilight didn't seem to know whether to shrink back in fear or hug her so hard she died.
"Th... thank you." Twilight's magic reached over the cylinder, and she pulled magical tools to her side and began to tinker once more. "The masters will be so very happy to see this."
Starlight raised an eyebrow, but inched away from the table in case she was about to ask the wrong question. "The masters?"
"Oh yes, the masters. They are so very kind." Starlight reflexively checked for any horrific scars that Twilight might be hiding, but she seemed basically untouched. "When I was cast aside by the unicorns for not being able to hatch a dragon egg," Twilight ground her teeth together at that, "they invited me across the sea, to this very exclusive academy. Until you showed up, I was the only pony to ever have learned here." There was a hint of jealousy there, and Starlight was quick to fix it.
"No, I'm not a student. I'm... well, if I'm honest, I'm a time traveler with a very poor sense of direction. I'll be honest, I don't even know where or when I am."
It was always a long shot, telling ponies that. Most of the time, they called her crazy. Other times, it just seemed like the sort of excuse anypony would use to excuse having broken into the most secure location in the entire kingdom. But very rarely, they believed her, and were willing to explain their entire world as though she were a child.
Twilight Sparkle could generally be relied upon to be crazy enough to believe Starlight, and this time was no exception.
"Oh. Well, why didn't you say so? This is Gondwana, land of the zebras. As for when..." Twilight took on a very cruel grin. "Less than a week away from the assassination of Princess Celestia."
Starlight cast a quick, nearly invisible spell to hide her paling skin. "A week away... before, or after?"
The grin grew wider. "Before."
Starlight couldn't stop herself from backing away. "Are... are you a traveler too?"
"No, of course not. I'm not good enough to manage that." Twilight gestured at the laboratory table beneath her hooves. "I simply designed the instrument of her destruction, and watched it be shipped off to our sleeper agent." Twilight Sparkle stood still, and smiled proud. "Mine are the hooves that have crafted the sword that will slay the gods."
"But Celestia isn't a god!" Starlight blurted out.
She hadn't thought it was physically possible for Twilight to smile wider. She had been wrong. "Then there shall be no doubt of our glory." With that, she turned back to the cylinder. "Now, the only question is, what will happen to you, traveler?"
"What do you mean?" Starlight asked.
"Will you try to stop us?" A few random bits of metal suddenly flew together under Twilight's magic, forming a jagged blade. "Will you go back in time and stop me from building the God's End? Or perhaps go back and try to convince me never to go with them in the first place?"
"No," Starlight forced herself to say. "I am not here to interfere. As long as there is one world where all is as it should be, then we may leave the rest to their folly."
"That sounded like you were quoting somepony," Twilight said in a half-interested tone, tapping the cylinder against her forehead with strange crackles of knowledge. "Who was it that said those words?"
"Me," Starlight Glimmer growled. "I was quoting me." Then she donned a grisly grin of her own. "You know, no matter how much you can squeeze into that cylinder, the equine brain can only hold so much. It's a failure."
Twilight just laughed. "Are you always this stupid compared to me?" She turned the cylinder in her magic, twirling it faster and faster. "It's not supposed to be passed around a campfire, being tapped against their skulls. It's supposed to turn an otherwise useless pony into the perfect military strategist." With that, Twilight Sparkle rammed the cylinder right through her skull, so hard and so fast that quite a lot of brain matter and skull fragments ended up on the floor.
Starlight Glimmer decided that now was the perfect time to leave and never return.