Primal Ban

According the the Heroes of Elemental Chaos Splatbook the Primal Ban is what keeps the World safer. The Primal Ban is the manifestation of a deep, older magic that lies beyond mere mortal awareness. Even the spirits whose existence keeps the Primal Bans churning have little sense of its true nature. As the Primal Spirits grew older and stronger, the Primal Ban began to grow with them and strengthen. As the Spirits became able to protect more and more of the Nentir Vale and the World they hedged out the rampaging Primordials and mitigating godly efforts to change the world. The Primal Ban is not a force field. The most powerful primordials and gods can resist or defy its effects when they want to put in the work. Many lesser beings can do so in places where elemental influence is naturally strong and match their nature. Or when the proper conditions are met. However, the surest way to get around the ban is to work through mortals. The warding does not stop mortals from making mistakes, so mortals who call upon divine aid or elemental energy can bring these forces into the world much more easily than gods or primordials can make their way in from outside.