The Celestials are immensely powerful beings, considered by some to be villains (confirmed as such in a few official works, where they have fought heroes and villains alike, they are greatly feared even by a being of Galactus' might - although in others Galactus has engaged and defeated them). This race currently consists of about 22 known Celestials, but the number varies considerably from story to story (in the beginning of time, for example, their were said to be as many Celestials as stars - which would of put them at a rate of trillions upon trillions (they were largely wiped out due to the First Firmament.)
Worshipped as "space gods" by many races, they may be benevolent by their own lights (unlikely as they often act robotic and rather malevolent, save perhaps to their creations, the Eternals), but they have absolutely no regard for the interests of anyone but themselves. They consider virtually all other beings 'germs' (much like the Monitors of DC multiverse), and destroy entire worlds and species if they think its necessary. Even the Watchers are not safe from their judgment.
Celestials are gigantic armor-clad humanoids. They are approximately 2,000 feet tall, some of them are even larger (such as Exitar, their executioner; usually 20,000 feet tall, he can seemingly increase his size at will, attaining planetary-level stature) : the otherworldly 'mad Celestials' can also merge with each other to create even larger and formidable forms.
Powers and Abilities
The powers of the Celestials are for all intents and purposes limitless. One thousand years ago, Odin - the head of one of Earth's most powerful pantheons - had decided to stand up to them. Working together with the heads of the other god families such as Zeus and Vishnu, he created a mighty weapon, the famous Asgardian Destroyer; when the Celestials reappeared on Earth, Odin animated the automaton with the life-force of all Asgardians except for Thor and confronted the space gods wielding the deadliest weapon in the world, the Odinsword. But the Celestials handily dealt with this ultimate attack as if it were a mere distraction.
The origins of the Celestials have always been vague, but the First Firmament revealed they were already present in Its structure, and disturbed Its order. For this, they went to war with Its preferred creations, the Aspirants, and in the conflict the First Firmament was ripped apart. The Celestials then moved to the new multiverse which was formed out of its substance.
Some sources believe the Celestials actually created the first life forms in the nascent multiverse, as well as, eventually, beings which would introduce death. These destroyers were themselves deemed a failure by the Celestials and sealed away in a pocket reality.
The Celestials would come under attack by the Aspirants at least once during the previous - seventh - iteration of the multiverse, and many of them were slain by the Aspirants' hyperweapon, the Godkiller.
It is known that they have visited Earth on at least four occasions (known as Hosts) during which they would alter and exterminate many of Earth's natives. They created both the Eternal and Deviant offshoots of humanity, as well as altering humanity itself. They were also responsible for sinking Lemuria underneath the Ocean when the Deviants rebelled.
They have done this on many worlds throughout the universe, experimenting with life using entire planets as petrie dishes and mercilessly pruning the life forms they deem unworthy or useless. The ultimate goal of all this remains unknown. Nevertheless, they are not entirely deaf to placation; Thor and Susan Storm have both communicated with the Celestials.
The Celestials, along with many of the Marvel Universe's abstracts, are killed by the Beyonders when these beings mount their attack on the entire multiverse. They fight the enigmatic aliens with all their power, but are almost all destroyed. In the miniature universe created by God Doom in the aftermath of the multiverse's destruction, no Celestials exist per se, but the role of the moon is filled by an orbiting Celestial head called 'Knowhere'. It is stated that the head belonged to a being who attempted to invade Doomworld, but was slain by God Doom.
After the multiverse is re-created, it is revealed a small number of Celestials had managed to evade destruction by going into hiding; however they are located and destroyed by the Logos, who declares they are no longer needed in the new universal structure. The Logos turns out to have become corrupted by the First Firmament, who seemingly finally had its ultimate revenge on the rebels; but one single Celestial, the One Above All, finds refuge with the Never Queen, the embodiment of potentiality. Since she embodies all unrealized possibilities, this allowed her to eventually accomodate the re-creation of all the Celestials from just him.
Notable Acts of Villainy
Although an amoral race of super-entities rather than true villains the Celestials have created dangerous artifacts and engaged in acts that some would consider immoral - usually to try and manipulate evolution in some fashion or to "purge" unwanted genetic codes from planets they are "progressing".
Creation of the Death Seed, a device which can reanimate the dead and render them into deadly servants of the Celestials' cause - or those of others.
Creation of the Black Vortex, a device which realizes its users' ultimate potential; this led to the destruction of the race which it was given to, and indirectly to the destruction of the Kree homeworld Hala.
Celestial technology is utilized by Apocalypse (though illegally so, since he was apparently punished by them over this over time).
Regular "purge" of worlds deemed "failures" of genetic manipulation (amounts to mass-genocide / planetary destruction)
As a side-effect of a war between Celestials and the Godheads of Earth the Destroyer armor was forged, which has been the cause of much devastation.
The creation of Terminus is a result of an act of vengeance by an alien race on the Celestials, who judged them "unfit" for existence.
A group of 'mad' Celestials from another reality encountered Galactus, who warned them he knew of their "true purpose" - this caused them to fly into a violent frenzy and try to destroy him, suggesting their true nature is sinister in nature.
Reed Richards of the Fantastic Four fought Celestials and managed to harm them with his advanced weaponry, this is one of the rare moments the Celestials have shown any weakness (save against omnipotent enemies such as the Beyonders).
The Celestials were said - by a Deviant - to have created the entire Deviant race just to consume en masse, although this seems unlikely to be true; they also mercilessly purged the Deviant race after they attacked their creators, and destroyed much of their numbers - making the Deviants an endangered species.
The Celestials have destroyed countless civilizations and worlds, putting their kill-count in the trillions... even Galactus, himself a planet-consuming entity, views the Celestials as dangerous.
They also created Mutants, thus in a sense the genetic war between humanity and these other "human variants" are part of the Celestials grand manipulation : they have also done similar manipulation across time and space, inspiring races such as the Kree and Skrull to do similar (a kind of "genetic arms-race" in which aliens create super-powered variants, often with cataclysmic results).
Marvel Cinematic Universe
The Celestials were known to appear in the Marvel Cinematic Universe, mostly in the Guardians of the Galaxy series. In the 2014 film Guardians of the Galaxy, a Celestial being known as Eson the Searcher was briefly shown using the power of an Infinity Stone to wipe out an entire planet civilization, though he would later lose the Stone afterwards. It was also revealed that a dead Celestial avatar's head was used as a mining colony before it became the space colony known as Knowhere (the home of the Collector), though it would later be destroyed by Thanos in Avengers: Infinity War.
In the 2017 sequel Guardians of the Galaxy Vol. 2, another Celestial named Ego appeared, revealing himself to be the biological father of Star-Lord. This may imply that Star-Lord is a Celestial hybrid, evident that he was able to briefly control the power of the Infinity Stone in order to destroy Ronan in the first film. However, Star-Lord soon learned that Ego himself intends to remake the universe in his own image by destroying all existence and that he was responsible for the death of Star-Lord's mother by implanting a brain tumor in her. Fortunately, Star-Lord got himself and the Guardians to foil Ego's plot by destroying Ego's core with a bomb made out of Sovereign Annulux batteries.
While the Celestials are known under this name ever since their introduction in the pages of 'Eternals', they never refer to themselves this way, and it may be Jack Kirby only meant for it to be a descriptive term, since it means nothing more than "beings from the stars".
On a side note, in may myths and games, Celestials were portrayed as heroic entities that came from the planes.
The apparent leader of the Celestials, the One Above All, is confusingly referred to by the same term as Marvel Comics' Supreme Being.