One cannot examine Marvel’s celestial origins without understanding the origins of Marvel’s Celestials, fittingly enough. The nigh-omnipotent beings have been around the Marvel Universe since long before the very beginning, and that’s not the last of the cosmic contradictions that define their story. The answers to simple questions like “How many Celestials are there?” or “What do they want?” are beyond the bounds of human comprehension. Put bluntly, the Celestials are responsible for the creation of the Marvel Multiverse itself, and for a not-inconsiderable amount of the beings that inhabit it - including the Eternals, the Deviants, mutants, and even Galactus the Devourer of Worlds himself.
The Celestials aren’t the most powerful beings in the Marvel Multiverse, but they’re well within the upper tier, and they’re definitely the most meddlesome of the bunch. That tendency has left their mighty fingerprints all over the deep history of Marvel Comics - and, in due time, the Marvel Cinematic Universe, as well.
The Celestials Were The First Beings Born In The First Firmament, A Sentient Universe That Predates All Others
To say the Celestials were there at the very beginning of the Marvel Universe isn’t hyperbole. They were the first beings born into the First Firmament - a sentient universe, the first universe of any kind to ever exist, and the only one to exist for quite some time. Lonely, the First Firmament eventually brought an incalculable number of several nigh-omnipotent entities into existence, demanding that they all serve and worship.
Of this initial collection, the mundane Aspirants were more than happy to dedicate themselves to the First Firmament. The multicolored Celestials, however, had other ideas.
Rebellious Celestials Created The Marvel Multiverse By Forming The Second Cosmos
In time, the Celestials fell under the influence of an entity known as the Fulcrum - possibly also the One Above All and maybe even an omnipotent Jack Kirby - who somehow ranked ahead of even the First Firmament in the cosmic pecking order and may have created it. The details of when, how, and why this happened are well beyond the bounds of human comprehension, and are best left vague.
Because of the Fulcrum’s influence, or perhaps because of something innate within themselves, the Celestials developed a desire to create life of their own, free from the oversight of the First Firmament. They wanted to observe a universe capable of growing, changing, and even dying. Creating wondrous technology and weaponry from the cosmic material of the First Firmament itself, the Celestials went after the Aspirants in a conflict that would last eons - and then they turned their attention to the First Firmament.
Detonating countless devices at the same time, the Celestials managed to shatter the First Firmament into pieces, splintering off endless new universes and marking the birth of the first - but not the last - Marvel Multiverse, which came to be known as the Second Cosmos. A broken First Firmament fled with the remaining Aspirants to the “Far Shore” of existence.
The Celestials Oversaw Transitions From The Second Cosmos To The Seventh And Current Iteration, Also Known As The Modern Marvel Multiverse
Working toward their own mysterious agenda, the Celestials colonized the Second Cosmos and the many universes within it, overseeing and manipulating the formation of life on endless worlds. Then, for one reason or another, the Second Cosmos came to an end - but the Celestials lived on, and soon initiated the Third Cosmos.
On and on it went over trillions of years, the Celestials seeing through Cosmoses Three through Five until the coming of the Sixth Cosmos, the one immediately preceding the (first) modern Marvel Multiverse. When that set of realities was consumed by the Black Winter, a multiversal storm, the Celestials brought about a new one. They watched as a single being named Galan survived the calamity and merged with the ashes of the Sixth Cosmos to become Galactus, one of many cosmic entities to populate the nascent Marvel Multiverse - the Seventh Cosmos.
But the Celestials, of course, had been around much, much longer.
They Went To War With Knull, The Primordial God Of Darkness, And One Of Them Got Decapitated, Forming Knowhere
The demise of the Sixth Cosmos led to the birth of the Seventh Cosmos, but in-between there was a darkness known only as the Abyss. The Celestials floated through it, doing whatever it was they needed to do to fashion a new multiverse - and it was there they encountered Knull, a primordial god of darkness.
Knull hated the life-creating Celestials the moment he laid eyes upon them, and so he created a sword of pure darkness and used it to decapitate one of them. The Celestials banished Knull to a purgatory for a time, but his sword would go on to empower Gorr the God Butcher and found the Klyntar race, AKA the symbiotes.
The severed Celestial head, meanwhile, became known as Knowhere Station, the site of numerous Guardians of the Galaxy-related adventures.