The Mare in Black fled across the desert and the Gunslinger followed.
Twilight Sparkle is the last gunslinger of Canterlot, itself a fallen city. The world has moved on. All that is left for Twilight is the all-consuming quest for the elusive Dark Tower at the heart of all creation. Few are those who have seen it, though all live on it's beam.
The White and the Black are at war, the Breakers gnaw at the Beams that prop up reality, and the wind of Ka--eternally cruel fate--is blowing. The Tower is near and Twilight will find it no matter what the cost.