During the Reflections comic ark, Twilight Sparkle and her friends discovered a reality that mirrored their own, with Celestia and Luna being evil tyrants and Sombra being a fair and just king. In an act of sacrifice, King Sombra took the evil from the two dark princesses and absorbed it into himself. Fortunately in "Of King's and Changelings" (fan-made comic, but it's the only good continuation out there so I incorporated it into my universe), Derpy the Wise and Duchess of love Chrysalis were able to free King Sombra and contain the evil within the Tree of Disharmony, capturing the Princess of Hatred, Princess Cadence to boot. And yet evil cannot be truly vanquished, it simply takes on new forms.

The mane six must tackle a new threat from this alternate universe. Themselves. When their counterparts threaten to throw both worlds out of balance using their own twisted version of the Elements, Twilight Sparkle and her friends must embark on another adventure into the mirror universe to save their friends and all they care about.