The only thing that can beat an AI is another AI. Unfortunately for Celestia, she isn't the only one the humans made and the others don't want ponies. Of course, if Celestia has her way that won't be the case for very long.
Certain alicorns have no regard for the rules of space and time. An astral sub-dimension becomes a potent weapon in what appears to be a divine prank war. Shining Armor hits the sauce.
A strange condition has Twilight courting death in the wake of her battle with Tirek. Now her life depends on Luna, and both lives will be affected. Hopefully for the best.
Right before the Elements struck, the Nightmare unleashed a spell out of desperation. A substitution spell. Otherwise known as the correct use of Kawarimi.
In an Equestria where Celestia and Luna have slain each other, a human is brought to Equestria in the body of King Sombra, in hopes that he might be able to save the kingdom from the fiends of Tartarus...