Title: Mirrorverse Mayhem!

Main characters: Twilight Sparkle, The Mane Six, Spike, Princess Celestia, Princess Luna, King Sombra.

Genre: Alternate Universe, Dark, Adventure.

Premise:
Evil Celestia’s attack on her sister ends up causing the barriers between the two worlds to shatter! With Evil Luna wounded and the dimensional distortions getting worse, Celestia takes action and enters the Mirrorverse to do battle with her Evil self! However, as the Sombra, Spike, and Mane Six attempt to fight along side her, Evil Celestia uses her sister as a shield, forcing the group to hold back. Seeing her chance, Evil Celestia fires a blast of magic at the Mane Six and sends them flying into the dimensional rifts that have formed, separating the group before she escapes into one of the rifts herself!

Now Celestia and Sombra must work together to find the Element wielders and stop Evil Celestia from merging the two worlds together!

Hmm. Well, I do think I'm going to add an Epilogue to The Glass Council, and that may be a long runner. We'll see.