The Light Prison

Twilight is corrupted by exposure to dark magic, and Celestia traps her in an extra-dimensional space called 'The Light Prison.' The Seam is a tear in space that is the exit to the Light Prison, but is usually invisible because the only thing that exists there is light.

This inspired me a little, when coloring -- [link]
It would be appropriate listening music.

Thank you for the EqD feature (653, #41).
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"Why are you here?" Twilight asked bitterly to the open air, her voice echoing about the endless twisted landscape.

From behind one of the many dark pillars in front of Twilight, an alabaster alicorn that had not even existed a few moments ago stepped out from hiding.

"Spike asked me to visit you." Celestia answered quietly, her voice flat and devoid of feeling. "Today is his birthday. Its what he wanted."

"So? You left me alone in here for two years and you only visit me because of him?" The unicorn snarled back, her voice quickly rising into a violent shout. Her eyes began to glow with a sinister magical light that grew in intensity with every hateful word. "What makes Spike so special? Why should he be more important to you than me? I thought I was special, that I was more than just another student to you, but you never cared about me!!"

As the last words left Twilight's mouth a spire of dark jagged rock burst forth from the ground between the two ponies, jutting sharply into the air far above their heads. Vine-like tendrils surged up from the broken ground to encircle the pillar before reaching out to curve and twist about the closest of the many that came before it.

Twilight startled back in shock, the burning light in her eyes winking out.. almost. Before she could even speak the growth of rock slowed and stopped, the sound of crackling rock filling the grove of spires as the vines petrified in place and became perfectly still.

Twilight quickly glanced back to her mentor, her mouth hanging open and her heart racing in her chest, but the alicorn had not reacted at all.

"Oh my gosh, I'm so sorry, Princess! I... I didn't mean to! Please don't be angry..." Twilight pleaded, desperation clouding her voice.

"It's gotten much worse here since I last saw you." Celestia spoke again, her quiet voice and as emotionless as when she first spoke.

"I'm sorry, I... I'm trying. But I can't... I can't control it anymore. I'm trying, but its so hard..." Twilight said, blinking away the tears that began to run down her cheeks. Twilight's lip quivered as some unspoken thought passed through her mind, and deep within her eyes the glow of light flickered like a candle burning low. She squeezed her eyes tightly shut and slumped to the ground, holding her head in both front hooves and shaking it back and forth slowly.

"How much time is left until you find the Seam and escape?"

"I wish I could go back. I just want to see them again..." Twilight muttered quietly to herself, her voice beginning to crack.

"Twilight..."

Twilight sobbed. Small streams of tears darkened her cheeks. "I don't know... I don't know..."

Some moments passed as Twilight lay crumpled on the floor. Celestia continued to merely stand where she had first appeared, no emotion ever coloring the marble face of the alicorn princess.

Twilight heard the rustle of cloth, and opened her eyes in time to see Celestia drape a large blanket across her. Celestia wore no saddlebag, so where it had come from Twilight could not tell.

"What... what is this?" Twilight muttered, her lip still trembling as she examined the blanket. It was plain, grey fabric, but soft and well stitched. Curling her front hooves together she pulled a fold of the material close to her face and inhaled the subtle but familiar scent that permeated the blanket.

"He wanted you to have it."

Twilight brought the fold back from her face and held it against her chest. Sure enough, the faint smell stirred memories within her. It was Spike.

"It smells like him... I remember it." Twilight said, her voice small and meek as she pondered those memories. She looked up hoping to catch a glimpse of Celestia's eyes, but the alicorn had already turned away to hide her face. Twilight's lips curved into a frown.

"Why is it I can see you this time? This is supposed to be your place. I never could see you here before even with all the pillars I made. It was always too bright."

Twilight waited for a response, but the princess stood there silently, unmoving save for her pearlescent ethereal mane rippling in the still air.

If the Empire is filled with hope and love, those things are reflected across all of Equestria. If hatred and fear take hold...

"Is it because you're... afraid?" Twilight asked hesitantly.

"...yes."

"Afraid of what?"

Celestia did not respond.

Twilight sat up from the floor, clutching the treasured blanket between her hooves as she stared at her mentor's back. The flame in her eyes had slowly grown, now once more the small waning trail of dark magical light that had bled from her eyes in almost every waking moment of the last few years of her life.

"You're leaving again, aren't you."

"Yes."

Twilight felt her heart sink as the Princess began to walk away. Her forelegs fell limply to her sides, the blanket slipping from her hooves and drifting to the floor. In the corner of the blanket, previously unnoticed by the unicorn, was a stitched letter R in fancy calligraphy. The thread was worn and sun-bleached from years of use, only the barest hints of its faded purple dye visible. The unicorns eyes grew wide.

"Princess Celestia, wait! What day is it outside?"

Celestia stopped in her stride for a brief moment, a shudder coursing through the alicorn's body, but still she continued to walk away. Her horn began to glow, its a muted yellow light glaring clashing harshly with the grey of Twilight's prison.

"It wasn't even close to his birthday when you put me here. It can't be his birthday! How old is Spike? What day is it outside?" Twilight repeated more insistently as she lept to her hooves and chased after her mentor. The flickering magical light streaming from her eyes began to flare brighter again, as worry and fear at her own suspicions began to grow within. "What's going on? Why won't you tell me?!"

Celestia still did not reply. Her body began to vanish, crumbling away into tiny motes of light that scattered like sand in an ocean wind.

"Please, don't just disappear again! Tell me! How old is Spike today?" Twilight cried out after her, glowing tears running down her cheeks again. The ground trembled and began to crack, lines crisscrossing the landscape all around the purple unicorn. Light streamed up from the breaks in the ground and then died as dozens upon dozens of small jagged spikes of rock began to inch their way up to the surface.

"Spike is 100 years old today, Twilight."