I am the greatest and most powerful unicorn that ever lived.

(Very small spoiler for Season 5 finale.)

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?

Twilight and her five friends are transported during a botched spell, across dimensions to... Ponyville, Equestria? But not the same one that they knew. Sitting on a bench is Harpsy and Babar. The Doctoress avoids apples from an aspiring Applebuck. And in a library across town, six stallions join their forces as they try to decipher the appearance of filly look-alikes of themselves around Ponyville!

Everypony has a story. Some tell of heroic feats and grand adventure. Others tell of villainy against the forces of Harmony. The story of an average guard, however, is one of obscurity. They live in the background, unnoticed but ever-present.

Silent Knight assumed his tale would be like that of his father's: join the Royal Guard and serve Equestria. He would be just another helmet and spear, part of the rank and file. And that was all he ever wanted.

However, when Captain Shining Armor asked for volunteers to form a House Guard for the recently returned Princess Luna, it was Silent's hooves that stepped forwards to set his new life in motion.