'The Catastrophe?'
Arthur didn't expect to hear about it so soon. He read and heard about it on Astria, the four Divine Beasts which attacked the small world and caused a lot of destruction but not enough to push all the races into extinction.
Zodiak mentioned that every world was attacked, but the power of the four Divine Beasts in every Realm varied. It depends on who's residing in that world, it's like an automatically prepared system that fixes the power of the invaders to match the overall power of the world.
For example, the Underworld, which could barely be considered a Medium-Realm is called a High-Realm because the behemoth is living there, and its strength is enough to raise the rank of the Realm. Moreover, when the catastrophe fell upon it, the strength of the Divine Beasts was unimaginable, far stronger than what attacked normal Medium-Realms.