The Celestials are an ancient race of entities that possess vast matter and energy manipulation abilities. They were present long before the dawn of the galactic communities and even the Asgardians. They were notorious for their utilization of the Infinity Stones.
Using the Power Stone
The Celestials originated at least several million years ago and have used immensely powerful tools to impose their will. One used the Power Stone as a means of destroying an entire planet. Knowhere, a space-borne mining colony, is actually the severed head of a deceased Celestial.
The Celestial known as Ego became aware of its own existence soon after his birth millions of years ago. After several years, Ego realized that he could manipulate molecules and began creating a planet around himself. After a few millennia, Ego began wondering if there was other life around him. He created a host body for himself and began searching the galaxy before finally finding life. In his travels throughout the galaxy, he planted seedlings upon thousands of planets, capable of terraforming them into new extensions of himself, but they could only be activated by the power of a second Celestial. He interacted with other species, impregnating many women in an attempt to create a new Celestial. Ego hired the Ravager Yondu Udonta to collect the children. However, none of the children could access the Celestial power, so Ego killed them.
Ego eventually met and fell in love with Meredith Quill. After Ego had killed Meredith, he paid Yondu Udonta to collect Peter Quill, but Yondu never delivered the boy, discovering what Ego had done to the other children he had delivered. Ego searched for his son ever since to see if he had inherited the Celestial gene.
The Celestials predate the dawn of the Asgardians and the Dark Elves and seem to be humanoid in nature. They have bodies that contain similar skeletal and neurological elements that humans hold -- but on a much larger scale. The biology of Ego seems to be unique to that of a traditional humanoid Celestial as he is essentially just a large brain with immense cosmic power that routinely alters the molecules around him in order to both protect the only vulnerable and physical component of his biology (his brain) and to mimic a humanoid form (or, presumably, whatever alien form that best suits his manipulative tactics).
The most that anyone has ever been seen of a Celestial are severed extremities and footage from the Collector that depicted Eson the Searcher using the Power Stone. However, it would come to be known to the galaxy that there are in fact living Celestials, as Ego, a planetary-based Celestial, attempted to transform all planets in the entire universe into his extensions, engaging in a massive battle with the Guardians of the Galaxy. Ego's cosmic powers made him capable of manipulating matter and energy at will and to a very extraordinary degree as well. Ego is also biologically immortal and invincible, as long as the "Light of his Planet" was alive.
As the Collector explained, their physical constitution is so immeasurably strong that they can wield the Infinity Stones. Celestials primarily wield the ability to manipulate and create matter on a molecular level, an ability that Ego utilized to create a host body for himself. There is only one known Celestial-Human hybrid, Peter Quill, who has shown the ability to draw power from his parent's essence, in order to manipulate matter and energy. Peter Quill was also immortal and could also wield the Power Stone. Peter Quill's powers and immortality seemingly disappeared when he killed his father, Ego.
Although another Celestial can be created through sexual reproduction it is extremely rare. The Celestial Ego fathered numerous children in the hopes of creating another Celestial but only Peter showed any signs that he inherited the genetics needed to be a Celestial.
- In the comics, the Celestials were deeply involved in humanity's early development, and were responsible for the creation of the mutant X-gene, the Deviants and the Eternals.