"What is it you'd wish for, Pinkie?"
"If I made a wish, it would be that Twilight was happy. That's all I want."
…
"You need to decide if you'd prefer the bliss of your ignorance or the pain of your understanding. Think carefully now."
"I want to know."
...
She peered into the vision as one peers through a window that is coated with condensation, squinting to make out the figures that were so close but obstructed. She raised a hoof and pressed it against the air in front of her, unsurprised when she met resistance. It was as though she was in a glass box watching a scene unfold before her, the ponies as actors and her and her guide as an audience in their seats.
"Can't you clear this up? I can barely make heads or tails of what's going on."
Discord chuckled, bending his long torso and bringing his head down to her level. "You can't see because you do not understand. And you do not understand because you cannot see."
Twilight scowled. This was one of the reasons she had been loath to ask the Lord of chaos for his help; he would never give her the answer she needed, but continually spoke in circles and riddles that made no sense.
"Maybe if you cleared it up I could see and understand."
Discord's shape melted down into a smaller form beside her, taking on the appearance of a shaggy-maned stallion, although all four of his limbs retained their original mishmash of species.
"Twilight, my dear, I am unable to do anything for you other than bring you here. You alone may decide to see, and you alone may understand just what you have lost. If you ask me you're better off not knowing."
Twilight snorted and turned her attention back to the scene before her. "I didn't ask you."
"Suit yourself."
Discord was being oddly amiable, and if she hadn't had a lot on her mind maybe Twilight would have been concerned. As it was, she focused solely on attempting to make sense of the fuzzy figures that danced just out of reach.
One of the shapes was a familiar shade of purple, and the longer she stared at this particular shape the more she recognized it. As the identity became clearer, she could hear its voice become less muffled.
"That's me!" Twilight gaped and prodded the glass with a hoof.
Discord chuckled, but said nothing.
As soon as she made the connection, her own figure became crystal clear and she could hear herself shouting.
"You made a huge mess of everything! You always do this… Why don't you just leave me alone for once?"
Twilight flicked her ears back. "I sound so angry." She tried to make sense of the pony her image was yelling at, but it was a vague pink blur and it seemed as though whenever her angry reflection would scream their name her voice would become muffled and incomprehensible. "Who is that?"
"Do you not remember?"
Twilight shook her head and pressed her muzzle against the glass in desperation. "No. Is that who I've lost?"
Discord nodded, but again said nothing.
She focused as hard as she could on the shape of the other pony, but she couldn't make out more than the two tones of pink, one on top of the other, and the fact that they seemed to be shaking under the verbal barrage.
Twilight forced herself to listen once more.
"All I want is a little quiet…! I don't want candy or cupcakes or company, I just want to be left alone!"
There was a quiet reply that Twilight couldn't hear despite straining her ears.
"No! This is more important than whatever asinine thing you've decided to throw a party for this time! I've no time to play around at Sugarcube Corner!"
Twilight shot a glance over to Discord who flashed her a wicked grin. "No time for fun and games, what an absolute boring Twilight Sparkle."
Twilight knit her eyebrows together and tapped her forehead with her hoof, chewing on her lower lip. Fun and games. Parties. Cupcakes. Each innocuous thing sent a small stab of pain through her skull as though they were trying to break through some kind of barrier. Those things should mean something to her. Those things meant a lot to the pony she couldn't remember.
She fell to her knees and pressed her face against the smooth surface of their glass box, groaning in frustration and pain as a headache lanced beams of pain through her head, focusing behind her eyes. Laughter. Something about laughing and giggling and…
She jumped up with a cry of pain, and she stared out at the scene again. Balloons. The pony's cutie mark stood out boldly against the rest of the figure's blurred outline: three balloons, blue and then yellow and then blue again. She felt as though her skull would split open from the pain, but she couldn't not know. She couldn't keep living feeling as though some vital piece of herself was missing, but never knowing what it was.
Pink fur, pink mane, balloons. Pinkie Pie.
It struck her like lightning, and all the sudden she could see the scene in its entirety. Twilight Sparkle's eyes were bloodshot and tired, and she had knocked a basket of sweets out of the pink pony's… Pinkie Pie's hooves. She was yelling with words laced with so much venom, and Pinkie was taking it without a word, eyes spilling over with tears, and lower lip trembling.
Pinkie raced out of the door, and the glass box followed her into the woods.
A shadowy figure.
A bargain.
Twilight struck the sides of the glass box with all her might and wailed, begging her not to do it, but it was to no avail. Hoof met paw, and a sinister laugh echoed through the forest.
The scene faded to black, and she was alone once more with Discord.
"Do you remember?" he asked.
"Bring her back."
Discord stretched himself back into his true shape and floated into the air. "I can't."
"Bring her back!" Twilight screamed, pounding the floor with her hoof as hard as she could. "Bring her back! Bring her back! Bring her back!" She collapsed to the ground, sobbing. "Please."
Discord shook his head. "I told you it'd be better if you didn't remember. Did you listen? Of course not, nopony listens to Discord. And now you must live with the memory of what you have lost. I hope you're happy."
Discord disappeared back from whence he'd come, breaking the spell and depositing Twilight Sparkle back on the grass of the Canterlot gardens with the frightened face of Discord's statue screaming down at her.
As she lay face down in the grass, she wished it would rain. But it didn't. She night was clear and calm, and the moon shone brightly and unobstructed in the the sky. Crickets chirped and the wind blew gently through her mane, carrying with it a haunting refrain of laughter from somewhere far far away.
A/N: The Prompt: I can't remember.