Takes place after episode two of season two, but before the two episode finale of season two.
The mane six defeated Discord, turned him back into a statue, and reharmonized the world. Fixing everything Discord changed. Ponies who might have been influenced by Discord's powers, and the few unfortunate ponies who had a portion of Discord's power put into them, were all turned back to normal, and other than a few guilty memories, or in the case of the latter, a bit of mild insanity, all was well.
Not everyone who had a taste of Discord's power disliked it however, and, in fact, one unicorn in particular, had asked for it.
No one knows what drove the unicorn to ask for such power, but once he had it the poor unicorn had gone insane, throwing a few spells around and casually warping the fabric of reality. One of those spells teleported him away from Ponyville before the Mane Six defeated Discord, and while the distance didn't stop him from being affected by the world's reharmonization, it didn't get rid of the chaos in him entirely.
It was so small, that with effort, the unicorn could have rid it himself. It wouldn't have been too hard. After all, his special talent was offering psychiatric help through joint lucid dreaming with his patients. It was a nice ability, but it left him bored and tired of reality.
So instead of using his knowledge of how the mind works and his ability to fix unwanted issues through lucid dreaming to rid himself of the small portion of chaos energy in his mind, he used it all to amplify it, learn from it, control it, and most of all, use it to bring his own dreams and desires into reality, and warp the minds of anyone asleep to make them his obedient minions. Normally this wouldn't be too much of a problem, one blast from the Elements of Harmony would rid him of the chaos energy even if he tried to hold onto it, and his powers aren't nearly as strong as Discords yet, but there is one area that he might have surpassed Discord in.
Subtlety.
Equestria needs help. Equestria needs YOU. You may not know why just yet, but you will soon enough. The question is: Do you have what it takes to help the Mane Six save the day? More Importantly: Do you want to? After all, if a unicorn can get a hold of Chaos energy strong enough to bring his dreams into reality, who is to say you couldn't do the same, with a little bit of help.
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NOTICE: The system I have set up for the story hasn't been tested yet, so comments, criticism, and ideas are all welcome and very much appreciated!