Story; http://www.fimfiction.net/story/419.....-Garden-Beyond

Beyond Equestria, in a place no Little Pony has ever known, Moon lived through many peaceful nights with her siblings Sun and the stars, away from the ruckus of the world below them. One fateful morning, however, she finds herself faced with the most dire challenge of her existence.

Despite terrible opposition and through the scars of loss and pain, she will need to discover her inner strength if she is to restore order once more, for her loved ones and the Little Ponies she cares so much about.

The tale of the imprisoned sister has been told and retold for centuries, but no Little Pony knows of the tale as it was seen by the forces beyond, by the beings of the Garden.
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The second cover I finished for the story late 2013.
Unfortunately, the chapter was never released, and I had completely forgotten about the picture until a few days ago.