Shiva and Kali
A Heaven-grade potency possessed the power to destroy a city. Although it was true that this depended on the defenses, as long as there was no defensive mechanism or power capable of threatening it, a Heaven-grade power was more than enough to cause dread among humans.
On the other hand, a God grade power possessed the might to defy humanity. Even the Human Emperor had to be cautious and careful with such beings since he himself was within such a power level. In short, a God-grade power possessed enough power to threaten the existence and prosperity of all humans.
As for Saint-grade powers... Such beings possessed the might to annihilate all of humanity completely. While it was true that the major cities had their own defensive barriers and attack arrays, an existence of such a level only needed time before wiping out everything.
On the Adastreia Continent, there were few Saint grade beings, but only one was all that was needed for humans to fall into chaos.