Within the MCU, the Eternals have secretly lived on Earth for over 7,000 years, but an unexpected tragedy following the events of Avengers: Endgame forces them out of hiding to defend humanity from the Deviants.
Much like the Guardians of the Galaxy were before their first movie came out in 2014, Eternals is one of the more obscure Marvel properties, with these characters descending from proto-huamanity, but boast incredibles powers and live significantly longer than humans thanks to Celestial experimentation, which also resulted in the creation of the Eternals’ arch-enemies, the Deviants.