Often touted as the strongest cosmic entity in Marvel Comic's exhaustively large cosmic pantheon, Galactus rightfully has nothing to fear. Except he does, especially when it comes to one of the Celestials featured in the MCU's recent Eternals film. Galactus has taken on and defeated many Celestials in the comics, but when he learns about Tiamut the Dreaming Celestial, it takes all his willpower not to flee to farthest reaches of the universe.
The 2021 film Eternals featured the MCU's version of the awakening and birth of Tiamut. In the movie Tiamut is grown from a Celestial seed buried within the Earth, feeding off the energy of the planet's lifeforms. Its destiny is to wait until the population reaches a certain threshold, at which point it will awaken and destroy the planet. While the Eternals are supposed to let the emergence happen, in the film they instead decide to stop it. It takes the combined effort the Eternals, Uni-Mind, and even the Dreaming Celestial himself to stop him from awakening and destroying the Earth.
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In the 2006 comic Eternals #6 by Neil Gaiman and John Romita Jr., the Earth is threatened by the imminent awakening of Tiamut. As the Eternals and members of the Avengers fight to stop this from happening, the universe shutters as the Dreaming Celestial prepares to awaken. When this happens, Uatu the Watcher cannot watch, and Galactus pauses and "remembers what it is to be afraid." Considering Galactus once beat multiple Celestials at once, so for there to be one that he is specifically afraid of speaks to Tiamut's incredible ability.
So, what is it about Tiamut that makes him so feared and powerful that even Galactus trembles in
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