The world of Equus is one of balance. Where there's peace, there must be unrest. Where there's happiness, there must be sorrow. That's how the world works.
Celestia has long sought to use her power to tip the balance in her favor, to forge a kingdom of harmony. Between her Alicorn might and the elements of harmony at her disposal, she's managed to create this kingdom.
But living in this kingdom of harmony brings distrust to the unknown. Little does Celestia know that this distrust may unravel everything she's worked so hard for.
The world will restore its balance, and it will take someone from an entirely different world to do it.