The Celestials are a godlike race from Marvel. They were the first beings created by the first Firmament, the first universe in the Marvel multiverse. Some of them industriously worshipped it, seeking its approval whenever they created life on their own, which prompted the First Firmament to name them "Aspirants". Others, the "multicolored rebels", who were "as many as the stars themselves", wanted their own creations to evolve, for the universe to grow, change and die. That was seen by the First Firmament as madness and sacrilege. A war broke out between the Celestials and the Aspirants for the sake of all of creation. The war greatly diminished both groups, desperate, who resorted to powerful weapons to get rid of their opponents, including the Aspirants' God-killer Armor that they used to win the final battle of the war.
After the final battle of the Celestial War, the Aspirants built their fleets and immediately entered a civil war, which gave the Celestials time to recover and crush their opponents.
The war ended by shattering the First Firmament into pieces. As the rebels detonated their weapons, hundreds of new universes split off from it. The First Firmament fled along with the remaining Aspirants, and the new universes formed a collective entity, the Second Cosmos and First Multiverse.
In opposition to the Celestials' apparent agenda were the Watchers. Having sworn an oath of non-interference in species younger than themselves, the Watchers found the Celestials' genetic engineering of such species to be the antithesis of what they believed. Thus, the Watchers and the Celestials were in pointless (as Watchers do not act) conflict for billions of years.
They created many races in the Marvel Universe including humanity. Through their experiments, their creations gained superpowers, which are now common in the Marvel Universe, thanks to the Celestials.