Every universe began with the explosion of an egg. The blast radiated energy and matter outward, spreading continuously.
This expansion process had no boundaries, but the total amount of energy and matter was limited. There was only so much, after all; it was merely a small fragment of the Third Firmament.
Once every last drop had been expelled, the Fourth Firmament collapsed from its core origin point.
The universe was destroyed as a result, and the Celestials had no choice but to emerge once more to clean up the mess. By then, they had become far stronger than before, and their understanding of all things had changed beyond recognition.
Thus, when shaping the Fifth Firmament, they added a new concept to the eggs in their incubation period. It was a valve meant to solve the problem of limitless expansion, placing a time limit on the future universe's expansion.
This concept was Eternity, born within the Fifth Firmament. In the end, it was merely one concept reclaimed from the many concepts of the First Firmament.
The Fifth Firmament existed longer than any other apart from the First Firmament. With all those restrictions added, it did indeed last longer. Yet, as more and more experimental species created by the Celestials consumed its resources, it was ultimately "eaten" hollow.
The multiverse died naturally, drawing everything within it back into the egg. Following the Celestials' arrangements, it began automatically reclaiming the matter and energy scattered throughout the universe, transforming into a "cosmic cocoon" and automatically incubating the Sixth Firmament.
The Celestials realized that unchecked internal depletion would not do either, so they helped the Sixth Firmament give birth to the strongest of the Concept Entities—Oblivion—to control the pace of depletion and expansion.
But Oblivion was too powerful. It was not particularly obedient and slacked