Shadow Bright, a mare born into a cult bent on realizing the ultimate embodiment of Nightmare. When this cult was taken down by Celestia, the foal Shadow was brought in to watch over. Ever fearful of what may happen, a close eye was kept on her, and an even closer one on her abilities. A foalish mistake from a rebelious teenager forever connects this mare to the Nightmare, darkness incarnate.
When she wakes up in a random ponies house, with no memory of how she got there or even where she was, it would seem that her destiny has caught up with her. Floating in the sky is the thing of her nightmares, something that means her attempts to separate herself from her destiny have failed. For when the sun and moon join in the sky, it forms the most powerful thing of darkness in Equestria.

An Eclipse.

"Half of the pony's pelt was dark white, the other pitch black. Her mane and tail were flowing in both directions, one liquid white, the other liquid purple. But when she opened her eyes I recoiled. One eye was fiery gold, the other slitted teal, and she spoke. She spoke with the voice of both Celestia and Luna. 'We are Nightmare Eclipse.' She yelled, her voice echoing through the castle. Nightmare Eclipse turned her head to me, and smirked. 'And you, must die.' "
The princesses have been corrupted, turned into a monstrosity, and all of Equestria has been poisoned by the Nightmare Energy. Now everyone is darker, eviler, and much more powerful. All but those select few who avoided it, and all this done by one unicorn named ShadowBright. Now she must quest to save the world from the Nightmare'd princesses, but she couldn't defeat them on her own before, how could she defeat them now that they have the Nightmare Energy on their side, even with help?