Almost everyone had never experienced a real Undead Calamity, and Negris was no exception. Its understanding of the Undead Calamity was probably just what it was like when Harvey conquered Holy Wall City in the Land of Fallen.
Even in the Star Burst Plane, the Undead Calamity launched by the Monarch was nothing special, because as long as one did not resist, the Undead would not attack the living.
However, that wasn't a true Undead Calamity.
Harvey's was merely the advent of a monarch, it was just that instead of corpses, he manipulated Skeleton Zombies with souls, which was essentially still the monarch's advent.
As for the Undead Calamity initiated by the Monarch, the goal was not to destroy but to conquer, which greatly preserved life, and so it seemed to lack destructive power.
But a real Undead Calamity was not like that. When the cadaverous Undead surged forth, everything would be eroded, corrupted, and defiled.