In the Beginning

In the beginning there was only one cosmos.

One universe.

One reality.

In the beginning there was only the First Firmament.

Although its universe was perfect, the First Firmament was consumed with loneliness. And so it created life.

These lifeforms, which it named Aspirants, served and worshipped it. Soon they made servants of their own. And the First Firmament was pleased.

But there were also rebellious Celestials, who wanted their creations to evolve and for the universe to evolve with them—so as to know growth and change and mortality.

The First Firmament saw this as madness and sacrilege. And so there was war.

In the final battle the First Firmament was shattered into pieces. Hundreds of new universes split off from it.

To preserve what remained of itself, the First Firmament fled to the farthest edges of existence, taking its remaining Aspirants with it.

Behind it, the new universes coalesced, forming a new collective entity. A second cosmos—and the first of a new breed.

The first multiverse.

    

This second cosmos was colonized by the Celestials. They evolved their own servitors, which they called Omegas—in opposition to the alpha that had been the First Firmament.

The second cosmos eventually grew old and died. But its death was the birth of yet a new cosmos. The multiverse was reborn again and again, evolving further with each iteration.

There was the Second Cosmos—first of the new breed, originator of the Omega Force.
The Third—the continuation, creator of Lifebringer One, the first hero.
The Fourth—the pilgrim, the true believer who journeys into mystery.
The Fifth—maker of magic, weaver of the illusions of change, home to the wizard Moridun.
And the Sixth—inventor of science, builder of the junction to everywhere.

And all the while the First Firmament watched from afar, plotting its revenge.

bibliography:

The First Firmament was introduced in Ultimates² #5 (2017). He told the history of the first six cosmos in Ultimates² #6 (2017). The identities of the six cosmos were fully revealed in Ultimates² #100 (2017). 
Moridun was introduced in New Avengers #2 (2015).

The cyclical nature of reality was also explained by Firefrost in Fantastic Four #230 (1981).

notes:

In Iron Man (2013) #13, Recorder 451 tells Tony Stark of a war fought between the Aspirants and the Celestials "for dominion of all existence at the beginning of time". He shows Stark an artifact left over from that war—the Godkiller armor, the Aspirants' ultimate weapon. However it is unclear whether he is referring to the war fought in the first cosmos or a similar one which took place in the current reality.

In The Last Annihilation: Wakanda #1 (Nov 2021) Black Panther theorizes that the exotic substance Vibranium was created during the multiversal rebirths.