As Equestria fell to the forces of Eggman, Princess Celestia had no choice to escape to another world using the crystal mirror. However the mirror sent her to the world of Apokolips, Darkseid’s domain.
After being tortured and brainwashed by Granny Goddess Celestia has now became a daughter of the Dark god and she is leading his armies to her world and to show Dr Eggman that he is nothing compared to Darkseid.