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.