Thought this will be the perfect theme for the story:

Dark Origin is a dark story about Dark Mist, a pony with two personalities(DS in medical terms).

Dark Mist isn't evil to begin with, she is caring and loving; but in her home village she is harassed and treated like a freak. This is because he has a second personality, Tainted Mist is sadistic in nature, but Dark Mist keeps this side suppressed the best she can. But when she is taken to a facility that promises to help her, she later finds out that this is not true, and is pushed too hard by Rosa Thorns, the lead director and commander of the facility.

But during training, Dark Mist is pushed too hard and loses control of her magic, the collar is destroyed; allowing the sadistic personality, Tainted Mist, to take over. Tainted Mist attacks Rosa as well as other staff members, Dark Mist then turns on her mother, who was there to observe the training. Dark Mist manages to fight her sadistic side, saving her mother; and returning to normal. Dark Mist escapes the facility, with blood on her hooves.

After escaping the facility, Dark mist returns to Trottingham and confronts Crimson Sketch, saying he should leave equestria with her. Crimson refuses hearing of what she did at the facility, rejected by her only friend; she loses herself to her sadistic side and becomes magically unstable; allowing magic to radiate from her causing major chaos around her. Dark Mist then becomes a force of powerful evil, Tainted Mist.

Magic

Dark Mist/Tainted Mist can use Dark magic, taught to her by Rosa Thorns, the lead director and commander of Project Dark Mist(name is a coincidence). The spells used by her can trap an opponent in their worst fears, taking away their will to fight or resist. Also used is the shadows around her, manipulating them to overwhelm her enemies, and most often sending them to a desolated illusion of the place they're in; be it Ponyville, Tower of the twin sisters, etc. Here she chooses to play games with them(imagine a cat playing with a mouse) to make them fear her even more. The more time they spend there, the more they become insane, Rosa learns that lesson all to well and her life is ended on the spot.

The dark magic also allows her to turn into a black mist, where she travels around the land. This is most used for intimidation or escape. Another magic she can use is earth magic, which she uses only to make statues of her victims(they become one);.wearing the fear ridden expressions.

Weapon

These daggers are the weapon of choice for Dark Mist/Tainted Mist, fast and lethal. she can use the shadows to aid in her fighting; allowing her to vanish and strike without warning. But she rarely uses this unless her opponent is pushing her back against a wall, its then that she will use them.

Goal

Dark Mist/Tainted Mist's goal is put everypony under an endless sleep spell, this puts them into a never ending nightmare for which they will never escape. Payback for harassing her and making her feel like a freak. being trapped in that darkness and sealed away makes her have animosity towards the Princesses and if she were to ever escape; they will meet their end,

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So that's the idea for Dark Origin, please let me know what you think below or go visit my fanficition.net page for the first chapter: