Actually, back in the Black Forest, Mephisto's power had not yet reached today's level. In front of Lord Withering Andesa, who controlled the Elemental Domain's Extreme Realm, he could barely hold his ground.
But after the battle with Andesa, his understanding of the Grey Domain deepened, and he unexpectedly broke through the Enlightenment Element Peak, transitioning into the Extreme Realm; in the history known to Brand, this unprecedented genius Sword Saint would only reach this level about five years later. With this progression, entering the Saint Domain in the grand era is not impossible for him.
Elemental Five Realms.
Enlightenment Element, Side of Truth, Peak Rules, Extreme Realm, Saint Domain.
Since the beginning of the first era, the rules of the world and magic, written upon 'Tiamat', have always stated such: after the Enlightenment Element, the five 'worlds' await — the traveler must cross each in turn to touch what Martha calls the 'Final Domain'.
