((Prompt: When one door closes, another opens.))
It was a very specific sort of dark in the room Twilight found herself in. The type of gloom that was too dim to make out any of her surroundings, but light enough to cast them all as vague shapes looming ominously over her. No matter how bright she lit her horn, Twilight failed to pierce the veil, resulting only in giving her night-blindness. It was as if the floor and walls outright refused to accept any light at all.
There was one thing she was certain of. This was not the library.
“Spike?” she called. Her voice did not echo; it fell dead against the darkness as though it needed light to be heard. “Owloyscious? Somepony?”
There was no reply. Oppressive silence smothered her like a blanket. Fingers of fear crept down Twilight's spine. Her coat itched and her tail swished back and forth anxiously.
She took a step forward and was surprised to hear the clop of her horseshoe echo against the stones in a way her voice had not. The sound rattled off of the half-seen walls, up and up, wandering like a lost soul as it told her of impossible heights, as though the room had no ceiling but shot forever into the sky.
Twilight looked up. There was a ceiling. Like the walls, she could just barely make out its shape a few feet above her.
She shook her head and steeled her nerves, ignoring the endless echoes as she made her way towards one of the walls. Twilight counted her steps, mapping out the layout in her head as she went. One, two, three, four, five, six, seven, eight. Eight steps.
The wall was hard and smooth. It felt like glass, and was cold to the touch. She searched up and down for any kind of distinguishing feature at all, but it was just a flat black plane jutting directly out of the floor. Twilight turned the way she had come, and counted her steps to the opposite wall.
One, two, three, four, five. Five steps? Twilight blinked. That didn't make any sense. It had taken her eight to get from the center of the room to the wall, but now she had crossed the entire room in only five? She looked back at the wall she had come from, and that was when she saw it.
A green crystal was embedded into a silver frame halfway between the floor and the ceiling. Though Twilight had not seen any sign of it before, the crystal was the only thing in the room that emitted light. Below, shimmering in that sickly sheen it cast, was a door.
Twilight slowly shook her head. “No...”
It wasn't the door she had found deep within Sombra's passage in the Crystal Palace; she had personally seen to it that door had been cleansed of all its magic during the reconstruction of the Empire. But it could have been that door's twin. It was cracked open just a sliver, and Twilight was certain she could see something moving in the blackness behind it.
“No!” she cried. She tried to channel a beam of light, to strike the crystal and neutralize it before it could fill her head with nightmares and visions of failure and loss, but no magic rose from her horn.
Twilight could only watch in horror as the door slowly, inexorably, drew open...
*
“Twilight! Twilight, wake up!” Twilight felt little claws pushing on her shoulder. Her eyes slowly fluttered open to see Spike standing over her, concern written over his face.
“S... Spike?” Twilight pushed herself up.
“You were having a nightmare, Twilight.” Spike patted her back. “It's okay, you're awake now.”
Twilight let out a gasp of relief. “Oh, thank goodness, Spike. I dreamt I wasn't in the library, but some dark room with a...” She trailed off as her eyes took in her surroundings.
She and Spike were in the center of a dark room, just gloomy enough to reduce the ceiling and walls to vague shapes. On the wall opposite them, the door stood, firmly shut. The crystal glowered at them.
Spike frowned. “Don't you remember, Twilight? King Sombra threw us down here when he seized the Crystal Heart and took over the empire.” His expression turned to one of optimism. “But we're not beaten yet, are we, Twilight? We just need to find a way out of here, then we can rescue your brother and Cadance, and the other girls, and then we'll kick his sorry butt back out to the glaciers, right?” He turned toward the door, tapping his chin with a claw. “It's gotta have something to do with this door...”