There was day, and there was night.
During the day, Princess Celestia had the sun raised, spilling warm light across the magical land of Equestria. While the sun was up, most ponies would be going about and doing various tasks, and jobs often related to their Cutie Mark. Whether the ponies ran a business, went to school, delivered mail, baked cookies, or took care of their family, there was no doubt that days for the ponies were busy.
When those tasks were done, the ponies would spend their free time doing various activities. Whether it'd be a casual sport outside or party with friends, or just quietly reading a book or doing a puzzle, there was an activity for everyone. None of these were extraordinary outside of powerful unicorns, pegasi of above average speed, or the strange kind of pony that casually defies reality on a whim. They were just normal magical ponies with normal days, and they often thanked Celestia for the light that shone above them.
However, work and play often tired the ponies out, and thus, when Celestia's sister, Princess Luna, raised the moon and brought about the night, the ponies would quietly tuck away in their beds, and sleep. And when they did so, their minds would travel to the realm known as the Dreamscape.
Luna walked among the doors in the Dreamscape at night, checking for any signs of distress from any of the doors. Should any of the doors rumble and glow, like the door was having a panic attack, that meant there was a nightmare to soothe. It was a one pony at a time thing, for the doors kept the ponies' dreams separate, and the dream avatars, containing the ponies' spirits, stayed with their bodies as they dreamt.
Merging dreams was taxing on her and only used for emergencies such as the Tantabus, and she was cautious in taking a pony outside their own door. Should she lose the pony and not be able to return them to their door within a week, their body would die. But those occasions where she had to do either were rare, and so her dream patrol was usually calm. So long as the dreams were separate and the souls didn't leave the doors, the ponies were safe.
On occasion, the little Dream Gremlins running around would mess things up by switching ponies' bodies. Luna hated catching the gremlins and putting them in their cage, then returning the souls to their rightful bodies. She was still annoyed by the time that they switched her sister with a party pony named Cheese Sandwich. She could not fathom why Celestia found it funny, since Cheese had made a fool of her.
However, as annoying as the Dream Gremlins were, she'd take rounding them up over having to fight their current imprisoned leader, Mara. Luna did not know how a small Dream Gremlin had obtained more power and evolved into a Dream Demon. She didn't even know how she learned the forbidden art of toxic magic, or how she could make her fellow Gremlins more powerful and deadly. Still, what she knew about Mara was enough to keep her in her prison deep in the dark recesses of the Dreamscape, so that she may never torment and kill innocent ponies again.
She decided to go check on the prison, make sure the cocoon was intact. It took a little flying and prancing on the star path, but she soon made it to the dark cave, which existed outside of anypony's dreams. When she landed at the entrance, she felt a breeze blow out of it, and a chill ran up her spine. She took a deep breath and went in.
As she walked through the cavern, the memories of when she was there last came back to her. She could still remember Celestia suffering under the effects of toxic magic before Starswirl had removed it from her. She recalled the innocent merged dream scene she came across that had been savagely interrupted by monsters and living nightmares. She could still feel the strains and sweat that had come as she battled the monsters, making the Dreamscape feel too much like the waking world. And the image of the one mare bleeding out from the neck after a giant squirrel bit her still haunted her.
She made it to the large chamber deep in the cavern, and her first instinct was to look down on the floor. While it was dark now, she remembered it glowing a bright green, being a window to the prison of innocent ponies kidnapped in their sleep. She remembered the winds blowing them around, the lightning striking them to power their demonic prison warden, and the ghastly wails that had cried out over the Dreamscape.
She looked up to the ceiling, where the cocoon hung. As she took off and flew toward it, she remembered how Mara had bragged about imprisoning Celestia in the magical cocoon meant to imprison the demon. Then, there was the the battle between Luna and Mara. The magic beams that she fired at the demon, and the demon's attacks of fire. Mara slithering around as a puddle at several points, and Luna throwing her into walls or throwing the wall at her. And of course, the moment when the demon had infected her with the toxic magic.
Though blowing up the floor to release the souls, and riding them to reach Celestia was one good thing that came out of that, the years after the battle had not been kind to Luna. She had lost her ability to dreamwalk. She had suffered from her magic behaving erratically, from blowing things up to her telekinesis activating at the wrong times to plants dying all around her. She still cringed at the memory of her accidentally creating a tornado and destroying a town. And then... there were the wretched nightmares, the spasms of pain, and the ever growing insanity.
And even when she regained control over her powers, even regaining control of the moon, her dreamwalking abilities still didn't work. It wasn't until many years later, after a millennium banished on the moon as Nightmare Moon, that she finally got them back.
When she made it to the cocoon, she was shocked to discover a large, gaping hole in it. With wide eyes, she lit up her horn and shone a light within the prison, showing no trace of the demon. There were pictures on the walls, etched there by Mara's clawed finger. The only one she could make out was a drawing beside the door of a stupid looking dream gremlin.
She wanted to take a closer look, but she pushed herself away from the cocoon, and decided to search the chamber in case there was a trap. She flew around the cave, searching every nook and cranny of the chamber. She searched the holes in the wall, and she searched the floor. She searched the ceiling and its cracks. She scanned every inch of the chamber until she flew back up to the cocoon. She wondered if Mara had already gone out through the exit to begin her reign of terror.
How am I supposed to track her? she wondered. After a moment of thought, she figured that maybe those etchings on the cocoon walls will give her a clue. So, she flew back up to the cocoon, looked at its ceiling, walls, and floor for anything unusual, and entered through the hole.
Once inside, she found that the inside was cold. The floor made a squishing sound as she stepped, and her coat was already gathering frost. She felt her eyes grow heavy, but she shook her head, as falling asleep in the Dreamscape meant that her body would be in a coma. So, she pressed on, curious about the etchings on the wall.
Luna drew close enough to them, and attempted to decipher them. There was an image of alicorns, with one looking as if she was falling, while the other was in a prison crying. Another image had one alicorn slashed and the other hanging by chained hooves and wings. And yet another depicted one alicorn decapitated and the other having lost her horn and wings.
She grew uneasy with each picture, especially since the alicorn that was always dying had a moon cutie mark, while the other one that was always distressed over her sister's death had a sun. It didn't take long to piece together who was who, and exactly what Mara wanted to do.
The pictures lead to the end result of an etching of Mara and other dream gremlins with puppet strings, in a manner of controlling the subjects of Equestria. She backed up, her heart pounding and her brain working in panic to find the dream demon before she caused too much damage.
As she turned, however, she discovered that the drawing by the door was gone. Sitting in the entrance was a blue creature, bearing hair that was similar to Luna's and a upright body shaped like an hourglass, clothed in a black, strapless dress. One clawed hand rested in her lap, while the other was daintily stroking the edges that had just begun to regenerate. Long spikes jutted from her elbows, and the hem of her skirt was bathed in a dark mist. Finally, the creature's noseless face turned to Luna, her red eyes glimmering and her black lips forming into a wicked smile.
With a giggle, she asked, "So, Lulu, do you like my art?"
Luna blinked before she lowered her head and stomped her hoof. Then, as she sprinted forward, she roared, "Whatever you are planning, I will not let you succeed!"
Mara yawned, and a large ball of toxic magic erupted from her palm. "Yeah, I don't have time to fight you." And with that, she casually tossed the ball inside the cocoon and then stroked the edges of the hole to speed up the cocoon's regeneration and draw closer to the entrance's disappearance.
Luna slowed down to duck under the toxic magic ball, and kept shooting forward as the magic splattered all over the wall behind her. Her heart raced, her gut twisted, and she could feel sweat on her forehead as she continued to dash for the exit. It was closing inch by inch, and Luna had precious little time to escape.
Unfortunately, she crashed into a newly completed cocoon wall, and she rebounded back onto the floor of the cocoon. Her head throbbing, she cradled it as she processed what had happened. She looked up at the wall, and her heart sank as she remembered that the cocoon was unbreakable from the inside. Her breaths appeared in the frigid air, and she slowly shook her head.
"No..." Tears filled her eyes as she stood back up and blasted the wall with magic. The resulting explosion didn't even leave a scratch. "No, no, no!" With a roaring neigh, she reared up and slammed her front hooves on the wall. With hot snorts coming from her nostrils, she turned and started to buck at it. Her hooves pounded on the wall as hard as she could, even as she was shivering from the cold and her yells were accompanied by yawns.
From outside of the cocoon, Mara laughed. "Sorry, Lulu, but you're on time out. Unless my finger or the horn of a unicorn touches this prison, you're not getting out of here! Besides, more than a thousand years was plenty of time for me to rot in there."
Luna had stopped bucking, and she turned around and tried to blast the wall with magic again. As it had the last time, it failed, instead pushing Luna away with a small explosion. She staggered back, hearing echoes in her numbing head. Shivering and feeling like ice, she murmured, "You will not win... Equestria has more valiant ponies than you can handle..."
Mara laughed once more. "Oh, really? Come on, I defeated the only one with power over dreams! I can handle a few pathetic little ponies..." She scratched her chin. "It would make for an interesting game to see if they can try to stop me."
Luna breathed through her teeth. "They will... I know they will..." And with that, Luna had fallen into her slumber.
Mara pressed her cheek on the cocoon and patted it in a mock loving manner. "Sleep tight, Princess. I'll take good care of the Dreamscape for you..."
She hopped down from the prison on the high ceiling, and drifted slowly to the floor. With her arms crossed and a smirk on her face, she said, "Yep, I'll take care of the Dreamscape all right. And I'll take good care of Equestria too."
She landed on the floor and gazed at the chamber's exit. "Now, where are my brothers and sisters?"
Knock knock knock.
"Luna? Luna, it's dawn, and you have not lowered the moon. Is everything alright?"
On hearing no answer from her dear sister, Princess Celestia knocked several more times before she decided to invade Luna's room. She opened the door, and strode in as she was greeted with various shades of blue from the walls, maroon curtains, the swirled pattern of the carpet, the soft lights perched on lamps shaped like barren trees, and the crescent moon which cradled Luna's bed.
Celestia stopped right in front of the round, white mat, where Luna had often meditated as she cast her dreamwalking spell. But what the sun princess found was her sister's limp form on the floor, lying in a manner most uncomfortable for sleep. There wasn't any indication of pain on the moon princess's face, instead bearing an eerie calm.
The first thing Celestia did was check Luna's vital signs, her own movements slightly quicker and shakier than normal. She quickly established that Luna's heart still beat and her pulse was normal, but no matter how much fuss Celestia was making, and no matter the noise that was quickly erupting from panicking ponies outside, Luna didn't even stir.
Did something happen in the Dreamscape? Celestia wondered.
Then, a guard appeared at the door. "Your Highness, is there something wrong?"
Celestia carefully lifted Luna with her magic. "Princess Luna has been afflicted with something that has made her unable to awaken." She put her sister in her bed, tucking her in gently. "Fetch the castle doctor, and have him figure out what's going on. In the meantime, I'll lower the moon."
The guard bowed, said, "As you wish," and left.
As Celestia left the room to perform the duty of switching the celestial bodies, she pondered on what could have stricken her sister. If the doctor couldn't determine a physical cause, perhaps this was a magical cause. Her mind went through the list of the hazards that Luna faced in the Dreamscape, which ranged from the dream eaters to the emotion-killing monsters, and from the nightmare weavers to the ever mischievous dream gremlins. Even so, Celestia knew that Luna wouldn't fall to those so easily--the creatures even feared the Princess of the Night.
Ultimately, Celestia's thoughts led to the final one, which had her feeling the phantom pains of a toxic magic infection from long ago, given to her and Luna by the very being that she was thinking of. The one being that Luna had imprisoned after a risky struggle, hidden away in the dark depths of the Dreamscape because she was so dangerous. And it shook Celestia to think that the monster was free, and posing a threat to Equestria yet again.
Mara.
Author's Note
And thus, my Dreamscape adventure fic begins! Oh, boy, have I been waiting to write this one!
Due to the limit on character tags, I wasn't able to get all the canon characters that were going to be in the main cast of the fic. This cast is a mix of canon characters and OCs of mine, and we'll be watching them try to escape a literal nightmare! Will they succeed?
Well... I oughta get the ponies in trouble first. 
