MovieBob did a multi-part series of video on the absolutely BONKERS comic book History of the Eternals...
But the concept, from what I understood, is that (some of) the Celestials (AKA Primordial Gods of the Marvel U) created the Eternals as unchanging 'perfect beings' and the Deviant as a rapidly reproducing and rapidly mutating race to see which one would basically "win"... and they got into a war. There's also an evil Celestial or two involved in there. I think the term 'Deviant' is given by the Eternals and they probably call themselves something else?
It's all very New Gods Kirby-esque pseudo-new-age-mythology stuff.