Celestial (first-fourth hosts), created capacity for mutation in pre-human anthropoids, involved in creation of Brethren.
The Celestials are a star-faring race of humanoid aliens who possess untold cosmic power. Standing two thousand feet tall, the Celestials are clad in full body armor. No Earth being has ever seen what they look like beneath their armor or knows their origin.
What is known is that they have visited the Earth at four different periods in the Earth's past, each time altering the course of history. The first Celestial Host came to Earth approximately one million years ago to perform genetic tests and experimentation on Earth's highest lifeform, the nascent human being. Testing the versatility of human genes, the First Host created two sub-species of humanity, the Eternals and the Deviants. Their sole legacy to the mainstream human race was the implantation of a dormant DNA complex which would one day permit benevolent mutations.
The Second Celestial Host came to Earth approximately twenty-five thousand years ago to inspect the results of their first visit. Finding the direction of Deviants technology counter-productive, the Celestials destroyed the Deviants' major stronghold, Lemuria. Repercussions of that destruction caused tectonic plate shifting that eventually contributed to the sinking of the continent of Atlantis.
The Third Celestial Host arrived on Earth one thousand years ago to inspect the progress of the human race. Their landing site was arranged by the Eternals working in conjunction with the Incas of Peru. The Third Host was met by a contingent of Earth's mythological gods, including Odin of the Asgardians and Zeus of the Olympians, who challenged the Celestials' right to interfere in Earth's affairs. The outcome of that encounter was that all of the major races of gods swore to forego their active involvement in the destiny of mankind.
The Fourth Celestial Host arrived in recent years in order to judge mankind's worthiness now that the dormant DNA complex for benevolent mutations had become activated by the worldwide increase in radiation levels. The Celestials deemed humanity fit to survive. A group of twelve human beings representing the great accomplishments of mankind accompanied the Celestials when they left Earth.
It is not known how many Celestials there are in existence. Even the number of Celestials on Earth in the Fourth Host is a mystery. Nine were known by name and function, others were glimpsed but not identified. The Fourth Host was led by the Celestial known only as The One Above All, who remained aboard the orbiting mother ship during the entire stay on Earth. The head of the landing party was Arishem.
However, it is known that the Celestials have visited many other worlds in order to perform genetic experimentation. For example, the Skrulls are known to be a result of Celestial experimentation. The Celestials have returned to judge many of these worlds, and Earth is apparently the only one of these that they judged favorably and hence did not destroy for posing a potential menace to the universe.
There have only been indications of the full scope of the Celestials' power. Each member of the Fourth Host withstood a full frontal attack by the collective power of the Eternals and the Asgardians. Arishem has been shown to have sufficient power to permanently seal dimensional portals to the godly realms. The Celestials are not indestructible, however. During the Third Host, the Celestials used their combined might to slay one of their brethren for breach of conduct.
Another indication of the Celestials' power is that when the Fourth Host left Earth, they eradicated all evidence of their recent and past presence from the minds and records of mortal men. Only the Eternals, Deviants, dimensional gods of Earth, and a handful of human beings are now aware of their existence and remember the nature of their visits.