The origins and true purpose of the Celestials can only be guessed at. They are known to have a special bond to Eternity, the embodiment of time, and have performed actions based upon Eternity’s will, but the scope of this relationship is unrevealed. At least one Celestial was born from the dying galaxy known as the “Black Galaxy,” but it is not known if this is typical of Celestial reproduction. (In the Earth X Universe, they reproduced by laying “eggs” within the core of planets, then empowering the lifeforms of that planet with the ability to protect the world - creating mutants - until the egg hatched and destroyed the planet in the process. Galactus fed on these “eggs” within planets.) Although the Celestials appear physically to be mere armor-clad giants, they exist beyond conventional understanding of space and time. They have also participated in many of the gatherings held by the Living Tribunal. At one time they created the race known as the Brethren from mere bacteria, sending them out to destroy races deemed unworthy, but they were finally driven from the Celestials’ service.
The young Celestials Devron the Experimenter and Gamiel the Manipulator were once responsible for monitoring Earth-78411 (“Dinosaur World”), but a petty argument between them escalated into pitting the Hulk and Devil Dinosaur against each other. The two were reassigned to monitor the Kree and Skrull races, and may have been involved with turning those races into mortal enemies.
The Celestials are most notorious for the Host’s four visits to Earth. The first host arrived one million years ago, releasing a swarm of Gatherer robots to collect DNA samples. The demonic Elder God Set sensed the purpose of the Celestials, knowing they would experiment upon primitive hominids. Set sent some of his semi-humanoid offspring, the Serpent Men, to curry favor with the Celestials to include them in their experiments, but the hominids saw through the Serpent Men and drove them off. When the Serpent Men attempted to appeal to the Host themselves, the Host destroyed them. Taking the hominids into their vessel, the Host the Host altered their evolutionary process so that three species sprang from the hominids rather than one. One species became known as the Eternals, endowed with cells charged with cosmic energy, which they learned to tap into over the ensuing centuries; another species was the Deviants, cursed with an unstable genetic code which caused them to mutate unpredictably with each generation; the final species was humanity itself, gifted with the potential for genetic mutation, a gift which would not become fully apparent until modern times.
The Celestials are known to have made similar experiments upon the Skrulls, Kree, Eyungs, and Gigantians, and likely many more. The Skrulls’ Deviant race ultimately overwhelmed the other two races, inspired by a Deviant from Armechadon named Tantalus, who caused Deviant uprisings on several worlds. The Deviant Skrulls are now the only Skrulls in the cosmos, save for one normal Skrull who escaped to Earth. The Kree Eternals are now extinct save for the warrior Ultimus. The Eyungs and gigantians destroyed each other in a war, but the Eyung Eternal Over-Mind still survives.
When the second Host returned to Earth around 18,000 B.C., the Deviants had become the dominant race on Earth, subjugating most of humanity, and ruling from the land Lemuria. The last outpost of human resistance in Atlantis had just been sunk in a volcanic eruption while attempting to drive out Lemurian invaders, and the area surrounding Atlantis was now in danger of further volcanic activity. When the Deviants beheld the Celestials’ mothership in the skies, they arrogantly opened fire on them. The Celestials responded by firing upon Lemuria, and, coupled with the preexisting instability, the entire continent of Mu sank, slaying millions of humans and Deviants. This event was known as the Great Cataclysm.
(Oops, I have to run out. I'll finish later.)