Celestia did what was best for Equestria. On the backs of human slaves, a kingdom of peace and harmony was built. She found out, too little too late, the price of arrogance.
Their Primarch reclaimed from the ranks of the enslaved human populace, the II Legiones Astartes, now a legion renewed, bears down upon Equestria at the center of a slave-backed empire of xenos.

A YHAY Inspired story.

Source for the original Canterlot artpiece that was used for the coverart can be found here.
The Celestia art used can be found here.