“Could you make eye contact when we’re talking, please? We’re friends, aren’t we?”
“Discord may trust you, but I don’t. You’re like a bomb—any moment you could bring Celestia down on us with a word from your pen. As far as I’m concerned, he’s a fool for keeping you so close.”
“I’m sorry. I didn’t ask him to do this.”
“He wasn’t always called Discord. That name was given to him by his enemies. I still don’t know why he insists on keeping it.”
“You know much about Discord before he was Discord?”
“Only stories. You know, I’ve never been to the zebra homelands. My brother holds the stories, but he’s never been to the homelands either. The stories came from our parents, and they got them from their parents, as far back as I know.”
“Yeah?”
“The whole region’s in turmoil now. Zebras fighting zebras fighting ponies. All because the Equestrians came and decided they knew how we should live better than we did. So I ask you: why is Discord the one with a name that means chaos and disharmony? Why not you?”
“It’s an interesting thought.”
“Yeah. Interesting.”
He looked off for a while, lost in his own thoughts.
“But lately, he’s been happier than I’ve ever seen him. I think it’s you. You’re what he’s always wanted: a student.”
“No kidding.”
“Yeah. From his perspective, everything’s upside down. The world is dominated by ponies who consider him a villain, but if he could find just one who could see his way of thinking, one to whom he could pass down what he knows … Well, he’s been a lot happier lately.”
“What about you? You seem to see his way of thinking. Why doesn’t he make you his student?”
“I don’t know. Maybe … Maybe he wants a pony on the inside. A pony he can send back to Celestia with a changed mind. A pony who could change the world.”
“And you think that’s me?”
“I don’t know. But don’t get too full of yourself. I still don’t trust you.”
He left.
Norse Pony critiques: The emphasis on dialogue is good practice, which every author should do. It has revealed some weaknesses in your technique which you should work on.
First, you did not identify the speakers with dialogue tags or attributions often enough, which forced me to go back to the start of the story and count lines forward to keep track of who was speaking. Attribution and tags are useful. I prefer attribution, myself, since I feel that describing body language adds a good deal to characterization. Unattributed dialogue can certainly work, but it requires a firm grip on the characters' voices so that it is obvious who is speaking just by how they speak.
That leads into a big issue in this story (and one which most authors suffer from to some degree, including me): your characters sound exactly the same. From clues in the story, I assume the "I" character is Twilight. But I was only able to deduce that from those clues, because that is not Twilight's voice. The fact that the nameless zebra and Twilight have identical voices makes Twilight's strange voice stand out even more.
You neglected to set the scene for us, either in literal scene-setting by telling us where the characters are and what they're doing, or by providing backstory for the setting of the story. That gives this story a disconnected feel which does it no service. It also makes it both confusing and unsatisfying when we realize things about the setting, such as that Twilight is working with Discord. Such revelations raise many, many questions: why? On Celestia's orders? In defiance of Celestia's orders? How is Discord free? Why is he STILL free? Discord is gathering a guerrilla army? For what purpose? Why would he trust Twilight? Etc. The lack of exposition denies us answers. I assume that a goodly bit of that is because you were practicing dialogue exclusively and so you did not want to include narrative blocks to set the scene. That means that you should practice expository dialogue while you are doing dialogue work. It's entirely possible to economically provide exposition in the course of dialogue. Note the pre-credits scene in the episode "Over A Barrel," for example. That skillfully establishes the premise of the episode in just a few lines of dialogue.
All-dialogue stories are very difficult, so you earn props for the effort. Keep at it, because it is an excellent way to hone your dialogue chops.
