"The stars will aid in her escape." That was the agreement. But Nightmare Moon did not live up to her end of the bargain. Now, the ponies she raised have escaped from the stars and want vengeance. So they break out and raise another of Celestia's relatives. One who was hated by his relatives far beyond what Nightmare Moon ever was. Morning Star.

Story four of The Truthseekerverse. The first to need an AU tag before publishing. Probably still possible to read before the others.