Right before the Elements struck, the Nightmare unleashed a spell out of desperation. A substitution spell. Otherwise known as the correct use of Kawarimi.
Nightmare Moon banished Celestia, only to be defeated by Ahuizotl. Ponykind were made the scapegoats to blame for her scourge. 1000 years they pray for a saviour to deliver them. Anypony will do. Enter the last hope, a pizza delivery dude.
In the humid air of warm Manehattan Nights, The Bluesmare's siren song calls from everywhere. A sundered white horse on an errand of peace and mercy follows the piper down the path of her life, towards a final act of redemption.
When a pony loses everything, it'll take the journey of a life time with help along the way to find out what really matters. This is that pony's quest to find what he is missing and to reconcile the past.