Living Gate

A Living Gate is a construct common to every universe which seals the universe away from the rest of the Multiverse. In fact, there is only one Living Gate, created by Diem himself; opening the gate in one universe opens it in all universes, and stepping through the gate allows one to step through to the "other side" in any universe of one's choosing. Of course, the Living Gate is necessary to keep universes and their inhabitants separated from each other (particularly X'dalzot and the aberrations), so opening it or travelling through it is strictly forbidden.

History
Diem first created the Living Gate when he created X'dalzot, as a sort of seal to prevent the aberrations from escaping their bubble of reality. When The God and The Goddess were tasked to create more universes, they too sealed their creations with the Living Gate, ensuring that the universes remained isolated from each other unless the Gate were opened.

Asmodeus's Pride
During the events of Asmodeus's Pride, the demon prince Asmodeus of Torven learned the truth about the Living Gate and the existence of the Multiverse beyond Torven. In order to meet The God and The Goddess and acquire godhood through them, Asmodeus shattered the Living Gate in order to escape Torven and enter The Dominion.

Shattering the Living Gate caused grave consequences across the Multiverse. The aberrations were now free to leave X'dalzot and roam into other universes at will. The fragments of the Living Gate flew into Golarion, where the keystone of the Living Gate plummeted to the ground and destroyed an entire continent. From then on, Golarion experienced massive meteor showers every night; shattered remains of the Living Gate falling through the sky. The keystone was recovered from the bottom of the ocean by a man named Aroden and became known as the Starstone, an artifact crafted by Diem himself with the power to grant godhood to anyone who touches it. Aroden thus ascended to become a god of Golarion, and eventually discovered the truth about the aberrations pouring into his world and others through the shattered Gate. He resolved to use his own divine spark to seal it shut again, sacrificing himself to create a patch over the hole in reality, a temporary fix that keeps the universes separate and the aberrations contained...for now.