Twilight was pacing. She'd been doing it a lot lately, and Spike was worried. He had no idea what was going on, or why Twilight was so worked up, but she'd been acting off for a week. Spike wasn't the only one who'd noticed, either. Starlight Glimmer, Twilight's friend and pupil, had also noticed that she'd been acting strangely. Particularly around her, and every time Starlight tried to talk to Twilight, there was always this uncomfortable and awkward tension between them. Starlight had began to wonder if she'd said something to make Twilight upset at her. The two of them were watching Twilight pacing in the library, deep in thought, from the door. They'd been sitting there for nearly an hour in silence, just watching the purple Alicorn walk back and forth in the room occasionally muttering inaudibly to herself. Finally, Starlight had had enough. She was tired of feeling like she might've offended Twilight in some way. She needed to know, once and for all, what had caused this tension between them. What was it that made Twilight so distant from everypony? Why was she just pacing? And most importantly... Why hasn't she slept? These were questions Starlight needed answered, and now she was going to get answers. One way or another.
With a sigh of determination, Starlight walked into the room. "Twilight," she said. "We need to talk."
The Alicorn initially gave no indication that she'd heard her student.
"Twilight!" Starlight shouted, finally snapping Twilight out of her trance.
"Hu-wha?" Twilight blurted out.
"I said, we need to talk." Starlight said firmly. "You've been acting weirdly all week, and it's gotta stop. I don't think you've even slept for several days, at least. What's going on? Have I offended you in some way? Is there a problem with me being here? If there is, you need to tell me."
Twilight was finally fully aware as to what was going on around her, and managed to summon enough brain power to respond. "No. It's not you. At least, I don't think it is." Twilight said heavilly.
"What's that supposed to mean?" Starlight inquired.
"I... I don't know." Said Twilight hesitantly. "Look, you've done nothing wrong toward me, and I'm sorry I've made you feel that way, and doubly so if I've made you feel uncomfortable around me. But to be honest... There's something that's been bothering me that involves you, though it's not been caused by you."
"I... I don't understand" Said Starlight, confusion filling her voice.
"I don't entirely understand myself." Said Twilight, before sighing heavilly. "I suppose it's time I tell you what's happened."
After that, she tells Starlight and Spike, who had since entered the room, about her nightmare the week before. About how she dreamed of executing Starlight. How in her dream, Starlight pleaded for her life, yet Twilight still did it anyway. She explained that it was this that caused the turmoil inside her. This is why she felt uncomfortable around Starlight. This was why she was pacing.
"Why don't you tell Princess Luna?" Spike chimed in. "She does have the ability to see into ponies' dreams and help them out if it's going bad. Tell her, and I'm sure she'll understand and know exactly what to do!"
"I've been thinking about that, Spike." Said Twilight slowly. "But I don't think it's a good idea to bother Princess Luna about this."
"Why not?" Asked Starlight. "This is all happening because of a dream. And like Spike said, dreams are Luna's domain. If anypony can help you, it's her."
Twilight remained silent in thought for a good couple minutes before she finally sighed. "Okay... I'll tell her. I'll write her a letter, and have Spike send it immediately."
Twilight, after she finished speaking, headed over to a writing desk, and inked a quill, before beginning to write. After a minute of scratching on partchment, She rolled the message up, and sealed it with a ribbon and hot wax before passing it to Spike, who in a breath of green flame, sent it on it's way. "It's done" Twilight said, with an air of finality. Now she just had to wait for a response.
Twilight didn't have to wait long, as less than ten minutes after she sent the note, Princess Luna herself teleported into Twilight's library. "Princess Luna!" Twilight exclaimed. "You really didn't have to come all the way from Canterlot because of my message. I was just wanting to know if you've heard of dreams this vivid."
"Yes I have, Twilight." Said Luna. "And it is for that reason that I have come. For you see, I believe this was no mere dream, but rather, something far more sinister."
"What do you mean?" Twilight asked, worry creeping into her voice.
"I had the exact same type of dream many years ago" Luna said. "One where it feels real, and you're able to remember it clearly after you've awoken. There can be only one explaination for this, and that is that you, Twilight Sparkle, have glimpsed into an alternate world. One where you capture Starlight Glimmer, and imprison her, rather than the events of what have transpired in our world."
"So... what you're saying;" Said Twilight slowly. "Is that what I saw... was real?"
"Yes. I believe so, Twilight." Said Luna softly. "However, I cannot be certain unless I see for myself. And so, I must ask you tonight, Twilight Sparkle, to help me to help you re-live this dream so that I may witness it also."
"How is that possible?" Asked Twilight. "How can you make me re-live the dream?"
"I can induce any kind of dream upon any pony, once they are asleep." Luna explained. "It is not difficult for me. However, the subject, you, must be willing, and if they focus on one particular type of dream, I can make that one happen. So, tonight when you lay to sleep, Twilight, I want you to fill your mind with everything you remember from this dream. The more you do, the easier it is for me to induce it."
Twilight agrees with Luna, and so they wait until it's time for her to go to sleep. As she slips into the covers, Luna's horn begins to softly glow with magic. A strand goes to Twilight's forehead, and her eyes slowly close.
After what only seemed like an instant, her eyes shot open, and she looked around. There she was, in the dungeons again. Flanked by the guards. But there was somepony else too. She turned her head to see behind her, and there, walking behind her, was Princess Luna.
"Be calm, Twilight." Luna said. "The inhabitants of your dream cannot see nor hear me. However I can see everything, and communicate with you. Just focus on what's happening, and try to do everything the same as before."
Twilight gives a small nod. One that was indecernable to anyone not looking for it. But as Luna was looking for some kind of reply, she saw it.
The rest of the dream played out exactly as before, and just like before, after the blade falls, and Starlight is executed, Twilight wakes up, and Luna walks over to her bed.
"It is as I thought, Twilight." She said. "What you have witnessed, did indeed happen in another world. I cannot say when this has happened, nor why you have seen it. But what I can tell you for certain that what you have witnessed was no mere dream. I must contemplate on this, and if I may, tell my sister."
"D-do you have to?" Twilight stuttered.
"Unfortunately, yes Twilight" Said Luna. "However, you have my word that neither myself or Celestia will tell anypony else"