The Beta Realm
The Beta Realm is the a remnant pocket of the multiverse that came before ours. Quite why it persists to this day behind its hardened dimensional walls (the Meggcarg) nobody born of our multiverse truly knows, but it's because of its continuing existence that we have the likes of the Book of Alternity, a seemingly-prophetic text which once translated in comparison to other artefacts details the events to occur over the course of our collective histories, albeit with room for interpretation assuming it's being read correctly to begin with.
Some of the species from that previous multiverse still live in the Beta Realm too, experiencing time as is done in the Outer Region in turn rather than like us in the Alpha Realm, however these beings are not the ones present who are of greatest note from the perspective of the rest of our reality. The Beta Realm was the site of the first real conflict in our iteration of existence, that event dubbed the Beta Incursion, wherein the Entities fought to fend off the Vyrol who were apparently seeking to enter from outside our reality, using that pocket dimension as their way inside. This took place before there was even time in the Alpha Realm. It was during the Beta Incursion that the walls around that part of reality really became reinforced, preventing any easy passage in or out of that part of space.