When the beam of Pure Harmony struck Princess Luna, she had expected to be banished or encased in stone, just like any other being who had threatened Equestria in the past.
She did not expect the Elements to tear her soul to shreds.
Neither did she expect to meet a father she had not seen in Millenia.
As the Universe calls out her name for aid, and as the Cosmic Voids threaten to tear apart all she holds dear, Luna is forced to make a choice: save Equestria and make right the wrongs against her, or defend Laniakea against the Eldritch Horrors that inhabit the Voids.
This story is a prequel to The Sentry, but can also be read as a stand alone fic.
I would also like to mention that the cover art was made by the wonderful BegasusTiuBe, who has some more incredible art works on DeviantArt.