Tap, tap, tap.
Celestia froze. No, nopony was behind her, she thought to herself. It's just the night. You've never felt comfortable in the night. The goddess sighed and hung her head, her mane flowing smoothly behind her. Though this time, the color seemed drained and not as brilliant as it usually looked in the sunlight. She wasn't usually out in the castle at night. Then again, she never was out at night.
She always (except now, of course) had slept trough the night with an uneasy peace, but now that Luna was back, everything was better now, right?
Right, she thought, a bit more confident this time. Luna could watch over the night that she rightfully deserved. Celestia could now rest easy, knowing her dear sister was up protecting her from the things that crept and crawled in the shadows.
Tap, tap, tap.
Then why did she still feel so uneasy? She looked over her shoulder and cringed. That infernal tapping... Celestia bit her bottom lip and gazed up at her beautifully crafted stained glass windows that shined with a beautiful, deep navy tint. She gazed up at the window that had the depiction of the defeat of Sombra carved elegantly into it. Bright blues clashed against deep, angry reds and was surrounded by ominous blacks and dark purples. The moonlight cast a cascade of azures and lavenders onto Celestia's ivory coat, and she let her eyes study the small and expertly crafted glass shards, admiring their precise cut. Despite the war it was portraying, it was breathtakingly beautiful.
The memory of it made her shiver, however beautiful it was. After that short-lived war, Luna had been so different, Celestia had noticed right away. Sombra may have been sealed away in the cold, hard polar ice, but his words echoed in Luna's head until--
Tap, tap, tap.
That blasted tapping again! She took a shaky breath. No... Celestia closed her eyes and gritted her teeth. No, she would not dwell on the past thousand years. Celestia let her hair droop in front of her eyes, her head sagging down to her hooves. One thousand cursed years of solitude had made Celestia jumpy and paranoid and painfully uneasy. Before the Nightmare Moon business, Celestia had never been afraid of monsters or creatures that would slink and scurry through the dark. But after the Nightmare, Celestia had barely felt safe in her own castle. She sighed, lifting her head back up to face the window.
Tap, tap, tap.
She sighed and turned around, walking slowly down the red carpet, still stiff and uneasy from the darkness that seemed to swirl and creep around her, jeering at her foalish fears. She tried to ignore the tapping that echoed around the hall, bouncing around her head as if to remind her that she might not be alone. She set out to find a more cheerful window. Perhaps the defeat of her Nightmare... maybe then she could be reminded of what wasn't a problem anymore. Once she had made her way down to the window after a few moments of silence met with soft hoofsteps, she gazed up at the desired window.
She hated that window. Even though Nightmare Moon was gone, Celestia still considered having the window removed from the hallway. She already had the guilt of banishing her sister to the moon, and she didn't need a blasted stain glass window to remind her of it. She didn't need it reminding Luna of it, either.
She frowned. Something was eerily off. It was so small of a detail, Celestia had to convince herself she wasn't imagining it. She unfolded her wings and flapped hard, lifting herself off of the ground and gliding up to the window, directly in front of Nightmare Moon's face.
In the original window had Nightmare Moon's mouth open in a scream, scared and terrifying. But now it was almost...
“Grinning?” Celestia whispered, placing a hoof on the window and running it down softly, squinting in her confusion. She slowed her flapping, lowering herself to the ground, away from the window. She landed softly on the velvet carpet, rubbing her eyes with her free hoof. Perhaps she was seeing things. After all, she hadn't slept in two days...
Despite her efforts to convince herself she was seeing things, she felt shock grip her legs and wings, and a white fear rang behind her eyes. What was it she just saw? She shook her head, turning away from the window and making her way further down the hallway.
She needed distraction. Something... happy and brighter. A window about something good, and something that didn't involve her sister. Perhaps Twilight's window would remind her of better times other than--
Celestia choked back a scream. Twilight's window was there in all of it's shimmering (and quite purple) glory, but the window had... changed. Twilight's eyes were no longer closed, and she was no longer smiling. They were open, and staring directly at the white mare with a red gaze. Twilight's unusual red eyes glared into Celestia's blue ones, menacing and cold. She began to back away from the window, fear and panic screaming in her brain to run away.
Tap, tap, tap.
“But...” she felt her flank push up against the freezing brick of the castle wall. She jerked her head up to look at the window behind her. Instead of Twilight and her six friends defeating Nightmare Moon, each pony was upside down and crying, the black pony with the night as her mane standing over them victoriously, unlike a moment ago. Celestia reared and galloped down the hallway, running in circles so as to not touch the shadows which were now unnaturally swirling and rearing like an angry horde of bulls.
Tap, tap, tap.
No, they weren't the only ones that changed, Celestia noticed, every window had changed... Discord was holding Luna and Celestia by their tails, Sombra was holding the crystal heart, Nightmare Moon holding Equestria in an eternal night...
What had happened to her precious windows? Celestia slowed down to a trot, then stood completely still. She gazed back up at Twilight's window, which was down the hallway, now. She felt her breath quicken into uneasy gasps as she walked back to the bottom of the window, her hoofsteps sounding like gunshots in the deathly quiet hallway. Her breath sounding like the angry winter winds in her ears.
The window was still so beautiful and expertly crafted, despite the horrid face Twilight was making. The moonlight reflected the myriad of purples onto her, a shot of red reflecting on the ground on before her. She looked over the window in shock.
Then she noticed what, no, who Twilight was standing over.
Celestia herself was beneath Twilight, her glory and regal stature diminished beneath the younger alicorn. Celestia froze. Was this the window of a coup? Was this the window of her defeat?
Tap, tap, tap.
She slowly walked towards the corrupted window, her steps echoing throughout the hollow and dark hallway. Her mouth hung open in paralyzing shock and her brain throbbed with a maddening numbness.
Tap, tap, tap.
Her breath quickened and her vision began to fog and blacken. She shook her head and stopped approaching the window.
What was this hell she had been cast into? Her mind screamed and whirled in circles, out of control and confused. What was this? What was this?
Tap, tap, tap.
Suddenly, the Twilight in the window unfolded her wings of glass, the glass rubbing together created a screaming screech, and Celestia let out a terrified scream. As if in response, the Twilight in the window pointed at her with a shrieking hiss that resembled claws on a chalkboard. Celestia screamed again and jumped back away from the window. The stained glass Twilight smiled, rows of glass teeth glittering in the ominous moonlight. An eerie laugh echoed throughout the hallway and the tapping echoed after it.
Celestia recognized that laugh.
Tap, tap, tap.
She heard where it was coming from.
Tap, tap, tap.
She knew where it was coming from.
Tap, tap, tap.
She had heard it before.
Tap, tap, tap.
Why?
Celestia spun around to face the window of none other than her own sister, Nightmare Moon.
The colored glass nightmare that was once her sister was tapping on the window it was trapped in, a crude smile on her face, her teeth pointing in a thousand directions, and her eyes as angry and as red as the gates of hell and Tartarus itself.
Celestia screamed in agony.
Tap, tap, tap.
The darkness continued to dance around the princess.
The tapping stopped.