"It was just a spell, a simple one at that. Well, until I took over Princess Luna's body by accident. Now I'm afraid that Equestria may fall to an unknown threat thanks to my blunder."

Souls are a fickle thing, easily corrupted or swayed by dark forces if ones will isn't strong enough. Princess Luna knows this better than anyone, even as her own soul shares its space with Twilight Sparkle.

With Equestria in its most peaceful era since the Unification of the Three Tribes, it remains unknown to threats from the 'Outside'. Princess Celestia leaves Twilight and Luna in charge without the knowledge of their predicament, to deal with the dragons. This leads both mares to find and eradicate an evil presence, before it unleashes an even bigger threat to Equus.
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-Mild Gore-
-Some Sexual Themes, but no Sex-
-Transdimensional Included-
-Anthro-