Queen Chrysalis, aka "Public Enemy Number One", is a dying mare. It has often been said that "In the most dire circumstances, the best solution will be discovered., and in her current situation she has come up with a plan that will not only alter Equestria's and the changeling race's paths, but the destiny for the Elements of Harmony.
This is set in an alternative timeline after the defeat of Tirek but before Thorax came into the picture and where the failed plan at the wedding cost Chrysalis more than anypony realized.