Arishem arrived to take Sersi, Kingo, and Phastos with him as a result of them stopping the birth of the Celestial Tiamut, which not only will bring consequences to the Eternals but also to Earth, as Arishem let them know Earth will be judged, leaving the big question of how he will judge Earth and if that means its possible destruction.
Engineered and created Celestial seeds and the process by which they form.
They are a faceless, voice-less, alienating presence, a cosmic metaphor for the implacability of natural disaster; and usually show up on a planet to judge whether its inhabitants are ready to have their energies absorbed or needed more time to cook.