While Phase 4 of the MCU is still largely shrouded in mystery and speculation, there has been a steady smattering of casting news revolving around one specific upcoming project: The Eternals, which so far, has reportedly nabbed big names like Angelina Jolie and Kumail Nanjiani as well as (potentially debunked) rumors of Stranger Things' Millie Bobby Brown as unconfirmed roles in the ensemble.
But major talent aside, the question still remains: Who--and, perhaps more importantly, what--are Marvel's Eternals? Where do they come from and how do they fit into the MCU's current landscape? Are they the next Avengers? The next Guardians of the Galaxy? The next Asgardians? Or are something else entirely? To get to the bottom of these questions, we need to take a look into some real-life superhero history.
The once and future King

It's impossible to disentangle the story of the Eternals from the legacy of one of comics biggest icons: Jack Kirby, the man responsible for co-creating the teams that established Marvel as we know it today like the Fantastic 4, X-Men, and Avengers. His name may not be as universally recognized as Stan Lee, but his fingerprints are all over the MCU and his ideas form the baseline DNA of our modern understanding of superheroes across the board. This is largely thanks to the fact that Kirby worked extensively on both Marvel and DC projects through most of what we know now as the Silver and Bronze ages--roughly between the early '60s to the late '70s.
Kirby's relationship with both Marvel and DC is a point of some historical contention, but the long and the short of it is that during his time at either company, he was largely given carte blanche to create the stories he wanted, which he would write, draw, and edit on his own. At DC, that manifested as the Fourth World saga, a new cosmic mythology that gave us characters like Mister Miracle, Darkseid, Steppenwolf (shout out to Justice League, I guess), and Big Barda. It was left unfinished after the books involved were canceled, though the characters and the stories were eventually picked up years later by other creative teams. The cancelation prompted Kirby to pack up and return to Marvel, where he did--well, essentially the same thing. The Eternals were Kirby's answer to the Fourth World for Marvel, and a chance to explore the same themes he began broaching over at DC in spite of his canceled books.
The themes themselves were pretty crazy. At this point in his career, Kirby was less interested in biff-bang-pow superheroics and more interested in things like high concept theology and how that would actually intersect with a world where superpowered gods walked the Earth. For the Fourth World, he took Judeo-Christian elements to make (and critique) a new, cosmic version of Heaven and Hell. For The Eternals, he went a bit more broad, cribbing from Greek, Roman, and Incan pantheons, as well as literary historical texts and epics--everything from The Odyssey to Shakespeare--to make a brand new creation myth for the Marvel Universe.