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.
A scenario where a being of chaos and war brought to a place of harmony and peace is not often a pretty one. However, the presence of a loved one can sometimes tame even the wildest of monsters. But for how long?