Rare, immortal beings, the Celestials were born from stardust in the earliest days of the universe.
A newly “born” Celestial is little more than a blob of plasma, somewhere in deep space. Overtime, they’ll absorb matter from the stardust around them, and older Celestials nearby will help to mould their form into something more usable, usually a small animal until there’s enough matter for a bipedal shape. The more mass a Celestial can absorb while developing and the types of elements they take in will determine their strength when fully grown. Living matter is a good source of these nutrients, so young Celestials tend to be voracious predators. Being the nursery of a young Celestial may spell an apocalypse for a planet with life, as all organic matter is consumed.
When a Celestial has enough mass to create an adult human or larger sized form, they’ll be considered mature.
Once a Celestial has fully matured, a more powerful one will assign them a space to guard- depending on their strength, this could be a room, a town, a city, country, a planet- even entire universes and solar systems if they’re strong enough. They'll be bound to this horizontal area for as long as it exists.
While the abilities of a Celestial depend on the individual, they’re often associated with the laws of space and time. A Celestial is incapable of dying, no matter what- even if they were turned to ashes and every individual particle placed somewhere far from the others, the pieces would find their way to each other and reform and heal. They''re capable of altering their physical form at will, using matter from their own constructed bodies and anything around them.
As mentioned above, Celestials vary in power, over a spectrum- the weakest being Violet Celestials and the strongest being Crimson Celestials. Crimson Celestials are godlike beings that tend to be the superiors of any nearby lesser ones. The very strongest Celestial? They keep to themselves. Most other Celestials revere them, calling this mythical figure The Prism, for they transcend the power spectrum.
To see a Celestial's true form would drive any non-celestial being insane. This is the reason why Celestials only leave the edges of space once they have enough matter to construct a solid form. However, without some part of their true form exposed on the skin of this cover, they'll be severely weakened, like they're sufforcating. To manage this they usually leave a small space- usually a single eye socket- open and exposing their true form.