In the blink of an eye, all life was wiped from the face of Equus by an abomination born of the stars. That is, all except for four immortal Princesses. Now they fight for the only thing they have left other than their grief... revenge.
One mare can only take so much. Even if she is an alicorn, something has to give sooner or later, and when it does, something terrible and all-consuming will be unleashed. Twilight has reached the breaking point and arcane retribution is imminent.