Well, that didn't take long. See, this is why you guys are awesome. I've received two suggestions already, but because they're both from anon, I can't know if they're the same person or not. Either way, I'm going with the Terminator suggestion first, largely because I know approximately nothing about Warhammer 40k. (please don't hate me.) So, without further ado, enjoy!
Spike strolled through the library, enjoying the peace of a day off. Twilight had decided to spend the day across the way at Sugarcube Corner, and he couldn't get enough of it. That was, until he heard yelling coming from just across the way where Twilight happened to be. "Aw, great."
By the time he'd made it down the stairs, Pinkie Pie was already there, pounding away on the library door. "Spike, come on! Something's wrong with Twilight!" Spike wasted no time, hopping onto Pinkie's back. The two of them dashed across the street, Spike skidding along the ground to Twilight's side. Her face was pressed into the corner of the building and she was muttering to herself, just repeating the same words over and over.
"Save John Connor. Save John Connor." Spike's hands trembled as he pulled a scroll and pen from his satchel. Quickly he jotted down a note.
Princess Celestia, Twilight needs help! I don't know what's happening!
He rolled the letter up and blew fire on it, drumming his fingers against the table as he awaited a response. He was exceptionally surprised to hear Celestia's voice from behind him.
"Twilight! Are you alright?" Luna was here now, too, and they both rushed to Twilight's side. The voice of authority that Twilight had grown to recognize seemed to snap her out of reverie somewhat.
"P-Princess...Celestia?"
"Yes, Twilight. I'm here. What do you see?" Twilight shuddered.
"There are...things with two legs." Luna and Celestia nodded.
"They're called humans. What are they doing?" Twilight shook her head, starting to mutter again.
"Save John Connor. Save John Connor." Celestia glanced over to Luna.
"We have to get in there. She'll need somepony to talk to once this is over." Luna nodded. Both of them leaned in, pressing the tips of their horns to the tip of Twilight's. Light flashed before them and they began to see what she saw, although they couldn't comfort her yet.
Twilight crouched in the corner of the room, just behind a pair of juvenile humans, one male and one female. A huge mass of metal had apparently crashed into the room, and another human was emerging from it, a female. "T-This can't be happening."
The male yelled in terror. "Come on! We gotta get this thing open! Avalon 412, put it in!" He drew from within his jacket a weapon, and began firing it directly into the other female. The bullets simply seemed to dissolve into her, leaving pockmarks of silver on the surface of her skin. Twilight buried her head in her hooves.
"No, no, no. You can't stop it. Nopony can." She had gone into shock from her first vision. She was jerked from this shock as the door began to raise and a buzzing sound was heard from outside the room. All three of them looked past the strange female to see yet another huge mass of metal crashing through the wall, smashing down on top of her and skidding along the ground even farther. The three of them jumped behind a concrete wall as munitions exploded and crates soared across the room.
As the vehicle finally skidded to a halt, another human walked out of the side, a huge male who's face seemed to be made of metal. His eyes focused directly on them and he spoke in a deep monotone. "I'm back." A buzzer sounded from the loudspeaker, signifying the emergency closure of the blast door they had just opened. The metal man rushed over, leaping underneath the door and pressing it open with his own body. He looked back at the group, increasing the volume of his voice. "GO NOW!" All three of them rushed over, Twilight skidding underneath the door easily with her small size. As she cleared the door, she glanced back and saw that the female was crawling out from underneath the vehicle, though the flesh had been burned from her face. She was now completely metal. Twilight ducked behind the corner, shuddering.
"These things...they're insane...they never stop..." The human female crawled under next, followed by the boy, who stopped halfway along. He glanced over to the huge man, nodding to him.
"Thank you."
"We'll meet again. Go!" The boy started through the door, but was stopped as the female, who had torn off both of her legs before Twilight had had the chance to turn away, grabbed ahold of his ankle. He screamed in pain, and Twilight cringed as crunches echoed through the crack in the door. Finally the larger man shoved one hand downward, tearing the female's arm away from his charge's leg. She turned to him, hissing in rage. Twilight shook her head as she flashed back to the Changeling invasion, reminded by the strange noises they constantly elicited. She was so wrapped up in her terror that she didn't notice when the two juvenile humans rushed down the hallway, intent on getting as far from the door as possible. She squeaked in terror as one metallic hand reached through the gap in the door, but it was withdrawn as the metal man grabbed ahold of the female's torn spinal wiring and dragged her back under the door with him. She disregarded his efforts, still trying to pull herself through the doorway.
Twilight retched in disgust as twice a new metallic face began to form on the front of the female's head, but each time it simply melted off like one's might in a nightmare. Finally, she skidded back underneath the door, screaming in utter rage at the huge man. He reached into his chest cavity, withdrawing a small cartridge which he shoved into her mouth, now holding the door open with his shoulders alone. Twilight got a shiver down her spine as she heard the last words uttered to the terrifying metal lady.
"You are TERMINATED!" Her entire world erupted in fire, and suddenly she was back. She lifted herself up from the corner she had been in, turning to Princess Celestia with tears in her eyes. Her face was a mess. Her eyes were puffy and reddened from crying in fear, and the words 'save John Connor' kept echoing in her ears. Finally Celestia wrapped one arm around her, embracing her in a warming hug.
"Celestia...they were terrible...what were they?"
"I cannot know, Twilight, but I do know that they are not of this world. You will never meet them in person, that much I guarantee."
"B-But it was so real!"
"I know, my dear. And we have much to discuss. I would suggest taking a bit of a rest for now. We can talk later." Celestia sighed as Twilight trotted off, her head lowered. "Something so violent for her first vision...I hope she can learn to cope." Luna made an uncertain expression.
"She'll have to."
Well, that was intense. Good grief, I think this one got away from me a little, but that's kind of what Terminator is meant to do. Thanks for the suggestion, anon, and keep them coming!