The Primordials

In the beginning, there was vast and formless nothing, known as the Void. Amidst eternal emptiness, three primordial beings emerged: Erathos, the essence of Order; Valyria, the essence of Chaos; and Nethras, the essence of Balance. In a moment of harmony, the beings came together in an event known as the Convergence. In this moment of Convergence, the beings created the Primal Material world of Myrathis.
Erathos, yearning for structure and order, created the landscapes of the world and the laws of its physics. Erathos sculpted mountains, carved the rivers, and filled the skies with celestial bodies; forging the structures that would abide these physical laws. Erathos' influence on Myrathis resulted in its extreme reflection in the Shadowfell, where all things must fall to the natural order.
Valyria, yearning for unpredictable chaos, breathed life into Myrathis, creating all the living things, from plants to animals to intelligent races. While life was subject to Erathos’ laws, Valyria gave all living things free will, the one thing in creation that cannot be bent to order. Valyria's influence on Myrathis resulted in its extreme reflection in the Feywild, where life flourished unfettered.
Nethras, observing the creations of the two siblings and finding them too extreme and at odds, wove the final element of creation: magic. A force that would enable creation to bend and break the rules of law and the chaos of free will.
Myrathis (and its extreme reflections in the Shadowfell, ruled by the Order of Erathos' will; and the Feywild, ruled by the Chaos of Valyria's will) would grow to form the roots of Astralys, the World Tree, from which many more realms would ultimately grow.
The primordials are believed to have flowed through the cosmis freely as Astralys thrived, until the Great Sundering and the subsequent Shattered War between the gods. This horrific conflict left many gods annihilated and claimed the lives of nearly three-quarters of all mortals. Recognizing the devastation, the primordials united in another moment of harmony, the Second Convergence. This critical moment led to the formation of the Godveil, a barrier safeguarding the material realms from both the gods of the Guardian Pantheon and the Usurper gods, as well as the Elemental Chaos that surrounds the roots of Astralys following the destruction of the Realm of Elements. There they remain to this day, pouring all of their essence into maintaining the woven barricade that protects the material realms from the drestructive powers of the realms beyond.
This barrier, formed and maintained from the very essence of the the primordials, serves to prevent any gods, Guardian and Usurper alike, from crossing over into the material realms. One of the only exceptions to this is the Eternal Arbiter, the only god capable of passing through the veil via the Ephemeral Crossing, the Realm between Realms. The second exception is Somnaria, the Dreamweaver, who resides in her santuary on Lunaria, the moon which orbits Myrathis. Somnaria is trapped on the material side of the Godveil as she recovers from her injuries sustained in the Shattered War. It is believed she is painstakingly rebuilding the Dreamweave from this realm, forever separated from her lover Gevren, the Forgehammer, who remains in their own divine realm of Moltana.

The Godveil
The more power a being is, the more difficult it is for them to cross the Godveil. Even beings that are not on the level of gods will still experience siginificantl difficulty with planar travel to the material realm if they are powerful enough to pose a substantial threat.