Warning! This article contains spoilers for Eternals! Read at your own risk!
Eternals is not what it seems on its face; there are a lot of surprises in between and just when everyone thought that it was the Deviants who were the villains, there is someone more sinister than them; someone who wanted to eradicate many for a Celestial to be born. In the case of Earth, it is Tiamut and he is the dreaming one. Who could he be?
Eternals: Who is Tiamut? Dreaming Celestial Explained.
The Emergence, as the new apocalypse of Earth, is the birth of a new Celestial. Arishem, the Judge, explained to Sersi that the Eternals are just robot-like-constructs who must protect the Celestial seeds on every planet against the attack from the Deviants. As it turns out, said Celestial seeds are feeding off of the energy of a living planet.
In theorizing about the creation of a Celestial, such could give birth to the different planets, galaxies, and suns. As described by Arishem, the destruction of a planet is the birth of a Celestial who could create, as if it is a cycle.
With the Eternals not agreeing with the plan of the Celestials, that Earth would be destroyed as a consequence of the birth of a Celestial, they made sure to save the day as Phastos created a Uni-Mind device, one which could combine all their powers and with the powers of Sersi, they were able to stop the Emergence from happening and they made sure that Tiamut would be a crystal from the moment he emerges from his place.
Tiamut remains to be a dormant Celestial. While they did not kill him, he remains crystallized for now. Being a dreaming Celestial, Tiamut is just waiting for the right moment to be born so he could make a new sun. In the comics, he was put under a million-year sleep because he attacked Arishem.
See how the Eternals prevented the emergence of Tiamut in the big screens now showing in theaters.