The title team was created by the Celestial Arishem and dispatched to Earth ages ago to protect it from the Deviants, their evil opposites, in the film Eternals, directed by Chloé Zhao. The Eternals stayed on Earth after defeating the Deviants to help humans progress, but they were not allowed to interfere in their battles. The true reason for this was revealed years later to the remaining team members: the Eternals were brought to Earth to prepare it for the birth of another Celestial, Tiamut, who would rise from the Earth's centre. Sersi (Gemma Chan) prevented Tiamut's emergence and saved the planet with the help of the "Uni-Mind," but that's not the only major event in Eternals.
The presence of the Eternals in the MCU should change the history of this universe (past, present, and future), but Marvel has ignored them, as well as the two most important moments from their film: Tiamut's failed emergence, which left his head and part of his hand in the middle of the ocean