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// Fear, Doubt, Distrust... Regret 
// Story: Truth in Shadows 
// by AutumnTale 
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It was a dark night, all but Princess Luna and the Thestral guards of the castle were asleep. Well them and another creature...

The Princess looked over her sky filled with stars and the glint of a crescent moon. It was a lovely night indeed. She knew many ponies admire it as she did. Maybe not as much as she did, but loved it none the less. Many songs and poems had arose with it beholding beauty, normal comparing a pony's shimmering eyes or deep coat. Luna adored reading the poems and songs. Nothing brought her greater joy than to have the acknowledgement of her subjects she shared with her sister. She was always glad that dreams transpire in her rule so she could watch over them.
The other creature loved the night as well. The cool air's embrace, the endless supply of shadows, and its sleeping ponies. It loved the sleeping equines, it loved viewing their dreams... And how easily they fell apart to its touch. Dreams were fragile, like souls or memories. All triggered by emotions, simple but complex things that all creatures had. It was this being's duty to disrupt the thoughts, trigger emotions to challenge a creature's mind... And it loved its job..

It moved from shadow to shadow avoiding all light. It did not light the light, not even the dim glow of the moon. It favored complete darkness. It relished in it. For what else could be more terrifying. It was the essence of its being. No pony knew its form but all had felt its presence. From the youngest foal, to the bravest stallion. No creature was safe from it. As it lurked through the small village, it was lead into a dark house. It lingered over the bed of a foal, a young filly. It found that the young creatures responded to fear best. A shadow was over the young foals face as a nightmare took hold filled with fearful, deadly creatures. The foal gave a whimper bringing joy to the menacing being before leaving. 
It slid through the shadows finding the next house to hold a strong, confident stallion. The creature looked over the pony, self-doubt had always worked on the strong willed creatures. It whispered stories of failures and troubles into the pony's soul, leaving its traces before the stallion shifted uneasily. The being murred to itself as it felt its essence grow. It left just as quietly as it came, leaving only doubt behind.
It came to the house of a young mare, only just out of home. The shadow loomed over the sleeping mare as it thought. Fear was to little, doubt would do to much... Distrust... The being placed images of lies and subtle hints into the young ponies mind. The mare mumbled unintelligibly as she began to toss and turn. The beast breath deeply but it was only silent as it left the room.

It spent most of the night, working in the shadows. Leaving fear, doubt, distrust. Nothing was safe, mare, stallion, filly, colt. All it crossed felt its presence, sensed its being, shared its pain. It had made its way over the land become more powerful like it did every night. It grew strong and feed on the darkness it left in its wake. It had grown stronger, bigger able not now move in the like as a liquid shadow across the ground. It had soon come upon a castle, it moved through it with great care. The guards awake only were giving shivers of discomfort. It couldn't afford to linger, its time had almost come. It come to a sleeping camber with a white sleeping mare. Her mane full of colour, her light coloured coat giving a dim glow. The creature knew this mare well. Regret was her greatest pain so it replayed the pony's greatest failure. Failure to save her sister. The creature had grow strong as the power fill it. The shadows of the room twisted and changed. They dragged over the walls slowly filling it with darkness.
Princess slowly entered the room as she did ever since her return to wake her sister. She ignored the shadow as they flicker.
She gentle nudged her, "Sister, it is time for the dawn."
She opened her warm purple eyes slowly as the shadows began to recede, "Hmmm already."
Luna only gave a soft smile and simple nod.
Celestia stood as the shadow became the shadow of a young foal. It had a face of sadness as it stood behind them on the far wall. It didn't like the end of the night and the shadows.
"Very well," Celestia spoke  softly as she moved out of the room.
Luna lingered looking at the shadow of the foal.
The dark princess spoke no longer than a whisper, "You shall always fade in the light when people are busy thinking of other things. But you shall always return... For with you, their would be no challenge in life. No true success, no bravery, no strength. Now come."
The shadow moved across the floor and into the shadow of the Princess. Luna accept the child shadow happily as she turned to go to her room. But not before watching her sister rise her sun into the world of the Equestria. The shadows is where it would rest the light away so it could once again play its part in the world. To serve fear, doubt, distrust and regret with the hope that courage, belief, trust, and comfort would take its place in the hearts of creatures. For what is one thing without its other.

So next time you feel a niggle in your brain, that slither of fear, shiver of doubt, fraction of distrust or even ache or regret. It is meant to be. For there would be no purpose without me... I am your challenge, your reason to keep trying. If you don't I get to stay... Forever lurking in the shadows of your mind...