Image/Marvel Studios
What exactly is the origin of the cosmic giants, the "Celestials," who appeared in the final trailer for "Eternals" recently? This time, let me give you a brief introduction.
At the very beginning of the Marvel Universe, there was the "First Firmament," which was itself a universe, and the only universe. Unable to endure loneliness, the First Firmament created its first servants to attend to it. Due to their unwavering loyalty, it named them the "Aspirants," the black and white Celestials; but among them also appeared colorful rebels, later known as the "Celestials." They wished for their creations to grow and evolve freely, and thus waged war against the First Firmament, who was displeased by this. Although the Aspirants built the colossal weapon "Godkiller" and slaughtered a large number of Celestials, ultimately, due to discord among the Aspirants themselves, the Celestials achieved victory, shattering the First Firmament. Thousands of fragments transformed into individual universes, and thus the Second Universe—the first "Multiverse"—was born.
Image/Marvel Comics
The Celestials began to create all things in the universe. They filled the cosmic space with light and matter. At the beginning of the Seventh Universe, this deeply offended Knull, the God of the Abyss, who resided in the Dark Kingdom. He then spawned a blade, severed the head of a Celestial, which later became the universally renowned "Knowhere."
The mighty Celestials are almost invincible, and no one has ever seen what they look like beneath their armor; currently, the only known weapons capable of damaging Celestial armor appear to be Thor's favored axe, Jarnbjorn, from his youth, and Knull's All-Black the Necrosword.
▼ Thor battles the mutant Apocalypse, who is wearing Celestial armor, with his axe Jarnbjorn.
After creating worlds, the Celestials, for reasons still unknown, traveled among the stars, modifying the indigenous intelligent life forms. This process would be divided into four stages, and they would visit each planet four times to judge their creations.
First Host
Is the first of four tests that a race modified by the Celestials will undergo. During this period, the Celestials, for reasons still unknown, modify the genetic structure of promising dominant species on various planets, differentiating them into three distinct branches: the primary race will exhibit normal physical attributes until some external catalyst triggers their latent genes, bestowing various different abilities upon individuals of that race, known on Earth as "Mutants"; the powerful race known as "Eternals" are usually endowed with cosmic energy; and races with unstable genes capable of prolific reproduction are called "Deviants." Over the years, the Celestials have conducted these tests on many races in the universe, such as Earthlings, the Kree, and the Skrulls, all of whom have their own Eternal and Deviant branches.
Second Host
Is the second of four tests that a race modified by the Celestials will undergo, occurring on Earth approximately 18,500 years BCE. At that time, the Celestials observed that the Eternals neglected their duty to protect humanity, and that the Deviants enslaved humans. To correct this imbalance, the Celestials intervened, destroying the Deviant base "Lemuria" and the vast majority of its population, triggering what is known as the "Great Cataclysm," which caused the cities of Atlantis and Lemuria to sink beneath the sea.
Third Host
Is the third of four tests that a race modified by the Celestials will undergo, occurring on Earth approximately 1,000 years BCE. This time, the Celestials' objective was to remove external influences from the worlds they experimented on. When the Celestials descended, they met with representatives from numerous Earth deities, including Odin of Asgard, Zeus of Olympus, and other immensely powerful "Skyfathers," threatening to close the passages between their dimensions and Earth. Ultimately, the Asgardian and Olympian gods, among others, had to agree not to interfere further with human civilization in the future, in order to preserve humanity's faith in them.
Fourth Host
Is the final test that a race modified by the Celestials will undergo. The Celestials will arrive to judge the race they created, determining the planet's worth. Earth's Fourth Host occurred in the 20th century, when they came to Earth to extensively collect information and samples. Odin, out of fear and rage, gathered all of Asgard's power to wage war against the Celestials, but was overwhelmingly defeated; however, ultimately, with the intervention of Earth's Elder Goddess Gaea, Odin, Zeus, Vishnu, and other Skyfathers successfully persuaded the Celestials of humanity's merits by offering them twelve "Young Gods." The Celestials were quite satisfied with Earth, erased humanity's memory of their existence, and departed.
▼ Facing the Fourth Host, the Destroyer armor, imbued with Asgardian power, the "Uni-Mind," formed by the combined strength of the Eternals, and Thor, fought together.
This scene in the "Eternals" trailer appears to be them using the Uni-Mind.
The Fulcrum
But even beings like the Celestials serve a master, an entity named "The Fulcrum." The existence and activities of all things in the universe are to serve this omnipotent being. For unknown reasons, it drives the Celestials to sow seeds throughout the universe, then allows the Eternals and Deviants to compete with each other. If, during the Fourth Host, a planet's energy signature resembles that of the Eternals, the Celestials will harvest the planet's energy and offer it to the Fulcrum; if the planet's energy signature resembles that of the Deviants, then the planetary harvesting machine, "The Horde," will come to end all life. The Fulcrum's only known purpose is to find an equal being to accompany it.
Normally, the Fulcrum always takes the form of a bartender named Jack, serving drinks and listening to their stories at "The Vestibule," a waystation where Eternals go after death. Many believe that the Fulcrum is actually the supreme being of the Marvel Universe, "One-Above-All," or at least an aspect of it; and the Fulcrum's appearance, like the One-Above-All, is modeled after the legendary Marvel comic artist Jack Kirby, who is also the creator of the Celestials himself.
Next, I will introduce several Celestial members with special status or stories.
One Above All
The leader of the Celestials, possessing the most powerful strength among them. Rarely appears in the public eye; even when visiting Earth, he allows his subordinates to actually descend onto the planet while he waits in his space mothership, and rarely interferes in other events. During the Fourth Host, he easily defeated Thor and the "Uni-Mind" (the combined power of all Eternals); and when the "Destroyer" armor, imbued with all of Asgard's power, battled the Celestials, he observed from afar on his mothership. At one point, when all Celestials except him were annihilated, he resurrected the entire Celestial team with the help of another cosmic entity, the "Queen of Nevers."
▼ After an intense battle with Thor, he showed Thor a vision of Odin kneeling before Arishem the Judge.
Many people confuse him with the supreme being of the Marvel Universe, "One-Above-All." The simple way to distinguish them is that one name has a hyphen, and the other does not! However, in recent years, Marvel has quietly changed the name of the supreme god to "Above-All-Others," possibly to avoid this embarrassing name collision.
Arishem the Judge
Arishem was the de facto leader of the Celestials during their four Hosts on Earth, with the duty of judging the planet's survival during the Fourth Host. As one of the first Celestials to create all things in the universe, he banished Knull to the void after Knull's first killing, unaware that it was Knull's home, ultimately allowing him to return and wage war against the Celestials. He was also the mastermind behind the planned massacre of the Deviants during Earth's Second Host, triggering the Great Cataclysm and causing sea levels to rise. He was also responsible for leading the Celestials in their hundreds of millions of years-long war against the Watchers. 
It is widely believed that, based on his iconic crimson coloring, one of the Celestials appearing in the trailer is Arishem the Judge. 
Nezarr the Calculator
Nezarr participated in all four of Earth's Hosts. During the Fourth Host, he appeared in Soviet territory, terrifying the army into attacking with missiles. In response, Nezarr created an illusion, making the firing soldiers believe the missiles flew back and killed them; this intense psychic shock caused all these soldiers to fall into an unconscious state. Later, during a battle with Asgard's Destroyer armor, one of his arms was severed, but he regrew the limb almost instantly, completely unharmed.
Jemiah the Analyzer
Jemiah is also one of the Celestial members who participated in all four Hosts. His duty is to conduct comprehensive analysis of collected samples, and he has excellent synergy and cooperation with another Celestial, "Gammenon the Gatherer." At one point, he left a "scripture" on Earth that no one except Ajak of the Eternals could read. The content described how the Skrulls, after conquering other races, also destroyed their gods, and mentioned several alien deities that no longer exist.
Based on his coloring, many speculate that he is the second Celestial to appear in the trailer.
Tiamut the Communicator/The Dreaming Celestial
Initially, Tiamut functioned as the communicator with the Fulcrum and was responsible for notifying the Horde to harvest planets. However, during Earth's Second Host, he was about to summon the Horde to harvest the world, but Arishem ordered him to stop and initiated the massacre of the Deviants. Believing Arishem to be malfunctioning, Tiamut then engaged him in battle, but was defeated by Arishem's combined efforts with several other Celestials and sealed beneath the mountains later known as Diablo.
Until modern times, the Deviants successfully awakened him, but he no longer wished to destroy Earth. Instead, he stood silently by the Golden Gate Bridge in San Francisco, observing all things on Earth, judging the world's value by his own standards. Although his chosen messenger, Makkari, was once killed, causing his internal mechanisms to automatically activate to reset the entire Earth; however, Sersi of the Eternals sacrificed her life to bring Makkari back, successfully preventing Earth's immediate destruction. Subsequently, guided by Uatu the Watcher, he discovered that he had developed self-will and emotions, and decided to ask the Fulcrum to spare Earth.
The Fulcrum asked him, "Why spare Earth?" Tiamut, removing his helmet, replied, "Because I care."
The very satisfied Fulcrum declared that Tiamut had evolved into an equal being, which was the goal it had been searching for all along. Finally, Tiamut accepted the Fulcrum's offer and departed with it.
Exitar the Executioner
As his name suggests, Exitar's duty is to be summoned to destroy all life on a planet when the Celestials declare it has failed the four Hosts. According to official explanations, the combined power or size of the entire Celestial team during the Fourth Host was far less than that of Exitar the Executioner. He once killed "The One," the strongest of the Watcher race, during a great war, but was subsequently killed when the Invisible Woman of the Fantastic Four damaged his brain. However, Uatu the Watcher stated that someone would soon fill his position, suggesting that it is merely a role or title.
Ashema the Listener
Ashema once had a special mission, ordered to take human form and bring Franklin Richards, the son of Mister Fantastic and Invisible Woman, back to the space mothership for evaluation. However, under Franklin's influence, she abandoned her original goal and stored an entire "Counter-Earth" created by Franklin within her body.
But during the Dreaming Celestial's attempt to break free from its seal by infiltrating the "Counter-Earth," Ashema was once completely controlled by the Dreaming Celestial. However, through the efforts of the Fantastic Four and Doctor Doom, she regained her freedom and re-sealed the Dreaming Celestial.
Eson the Searcher
The explorer and traveler among the Celestials, who came to Earth during the First and Fourth Hosts. During the Fourth Host, he observed the underground Deviant city of Lemuria, but was attacked by the Deviants; however, he casually absorbed all their energy and destroyed the entire city. At some point, he offered a deal to the mutant "Apocalypse," bestowing upon him the power of Celestial technology to shape the destiny of this world, but warned that they would one day return to collect payment for this gift. 
In the 2014 film "Guardians of the Galaxy," the Celestial shown in the footage displayed by the Collector to Star-Lord and others is Eson the Searcher. 
Progenitor
According to Loki's explanation, the Celestial known as the Progenitor arrived on Earth approximately four billion years ago, at which time it was infected by the Horde and on the verge of death. Before dying, its blood seeped into Earth, forever altering Earth's evolutionary path, making Earth a special existence even for the Celestials. To prevent the Horde from threatening the Celestials, the Celestials specifically used the Progenitor's genes during Earth's First Host to create the Eternals, Deviants, and the X-Gene, hoping they could become a force strong enough to resist the Horde in the future.
Celestial Madonna
The Celestial Madonna is a very special being. She descended in Luoyang, Han Dynasty, in 114 CE, and met the scientist Zhang Heng, a member of the "Brotherhood of the Shield." Carrying another Celestial embryo within her, she required a massive energy source to give birth to this child, but the process would also kill her. Regardless of whether Earth or the Moon was chosen as the nutrient source, it would ultimately lead to Earth's destruction. So Zhang Heng suggested another possibility—for her to go to the Sun, use its abundant energy to give birth to the child, and avoid Earth's destruction. She continuously absorbed energy within the Sun until 1956 CE, when Leonardo da Vinci, traveling through time, flew to the Sun to observe anomalies and she gave birth to the Star Child before dying.
Star Child
As his mother said, the Star Child is a "new/unique/forbidden" existence. Unlike the extremely complex language commonly used by the Celestials, the Star Child's language is mathematics. Since being retrieved from the Sun by Da Vinci, he gradually grew and learned within the Brotherhood of the Shield's stronghold until he discovered the secret research of Brotherhood Master Isaac Newton, "The Quiet Math," after which he suddenly went berserk and began to destroy everything. Finally, under the manipulation of the Human Machine, who possesses infinite possibilities, he and Newton arrived at the doomsday world of 2060, as foretold by The Quiet Math.
In the Marvel Universe, many stories and settings are constantly rewritten. Whether it's the origin of the universe or the Celestials, there are always numerous conflicting accounts. According to official interpretations, no single origin story for the Celestials is correct; rather, they are a manifestation of infinite possibilities. This seems to serve as an explanation for the inconsistent settings written by different authors over the years!
Text: Marthus