In the Divine War summoned by the World Secret Domain, the role that a single divine being can play is very limited. Therefore, while the Divine Lord was surprised that Ash could summon the Heart Sword Divine Being in such a dire situation, he did not take it to heart—what change could an additional divine being by Ash bring?
Indeed, this was true; with current resources, no matter how perfect the casting skills of the Mirror Demon were, he could not overturn the battle. Or to say, for the Mirror Demon to fight against three Divine Lords who command the World Secret Domain with such transformational sorcery, he already turned decay into magic.
