Who was the Celestial Tiamut from the movie Eternals? Well, the MCU did tweak a little bit on what he was, Arishem told the eternals that this was a new Celestial being born inside the Earth and that he needed to feed off of sentience orintelligence to be ready to emerge into the wider Universe.
IN the MCU, A Celestial seed was sent to the planet Earth millions of years ago, as directed by the Celestial Arishem. The seed fell like a meteor into the earth, burrowing deep into its core to rest. It would take a lot of energy from sentient lifeforms for it to grow, as with all Celestial births. The rising Celestial known as Tiamut would start to grow off of the energy of the primitive human race that inhabited its host planet, but the energy was insufficient to grow the Celestial due to the species' primitive status. The planet was also infested with Deviants, which made conditions worse. Arishem dispatched 10 Eternals to Earth in order to exterminate the Deviants and assure humanity's evolution and the birth of Tiamut.
But is this really true or just a piece of the information given by Arishem to the eternals in order to hide the true form and true nature of a celestial that even Galactus feared. Yes, this was one of the few beings in the Marvel Universe that brought fear into Galactus’ eyes, the same devourer of worlds, the one and only survivor of a previous universe. Imagine the power and status Tiamut really had in order to make the world eater feel fear. But this wasn’t what was shown in the MCU as it could have repercussions in the entire story telling of the cinematic universe, this was a being that even the Watcher, the same one from the What if series, couldn’t see him when he awoke.
In the comics, Deviants were the dominant force on Earth, and Tiamut the Dreaming Celestial was ready to call the Horde, another collective of beings meant to harvest planets. However, Arishem cancelled that order, and had many of the Deviants killed. The Dreaming Celestial deemed Arishem to be malfunctioning and tried to assume command by attacking Arishem. The Dreaming Celestial then found himself attacked by all the other Celestials, cause it took their combined might to subdue him, and he was imprisoned underneath the mountains. The other Celestials ripped his spirit from his body and placed in a device known as "The Vial," They then sealed the Dreaming Celestial's body in a vault under the Diablo Mountains in California and For millennia, the Celestial slept.
The Dreaming Celestial, he was one of the most powerful Celestials, once classified as Tiamut the Communicator with the function to alert the Fulcrum, a being who is essentially the creator god of the Marvel Universe, he was tasked to alert him of the findings of the Celestials during planetary examinations and send a signal to the Horde, and to collect the planet's aggregate life force to be transmitted to the Fulcrum once completed. In the end, when he awoke from his slumber, he decided to spare the earth against the will of the Fulcrum and so with this, he became the only celestial to break free of his programmed tasks and purpose and join the fulcrum as a companion to oversee and design the universe.