This is the first book of The Humanity Within Trilogy. Its sequel is What I've Done.

I wasn't always like this, a monster, running through the woods on an alien planet. I was once a man. I had dreams, a girlfriend, and a great job.
Then They came. In the middle of the night, They scooped me from my home, and... changed me.

Now I'm a beast, a demon the like of which the world I've become stranded on has never seen before, scraping by on the fringes of society, hiding from the natives, and doing my best to avoid notice.

After all, who could ever be friends with a monster?

Cover Image done by the very talented PaintSplotch! You can find his FimFic account here.
And here is his DeviantArt Account!
Now 100% approved by Twilight's Library!
Mad props to my prereaders:
ThatPonyWithASword.
Admiral Applejack.
JXWheeler

In a feat of jealous rage, King Sombra kidnapped Princess Skyla from her crystal cradle, and took her to a faraway land. She was supposed to be his twisted legacy, his way to reclaim the Crystal Empire, a mere tool to exact his vengeance on the Crystal Princess. She was never meant to become his daughter.

Unhappy that her future as a member of the monarchy is denied by Celestia, Sunset Shimmer escapes through Starswirl's mirror into another world, planning to take it over to prove once and for all she was fit to rule. What she didn't expect—besides a species change—was to arrive in a world already conquered.

TvTropes Page (Just created! Needs Help!)

Art (Will contain spoilers)

Sunset Shimmer - Bloody Sunset

Spoilers Ch. 11+: Angel - Angel and Sunny
Spoilers Ch. 18+: Princess - Comic
Spoilers Ch. 121+: Alejandra Acevedo - Flying at the speed of death
Spoilers Ch. 161+: Apple Bloom - Bloom
Spoilers Ch. 180+: Food - Sunset's Party Dress
Spoilers Ch. 185+: Fugue - Fugue by Rametep
Spoilers Ch. 197+: Exhausted

(Art is always welcome!)

WARNING: CONTAINS SPOILERS FOR THE WHOLE STORY: TIMELINE

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?