It’s funny to look back at 2012’s The Avengers and how the most epic blockbuster movie crossover ever (at the time) now seems almost quaint after experiencing the mindblowing scope of Avengers: Infinity War.
These days, the question that dogs the MCU isn’t “how much longer can this go on?” as much as it’s “how much bigger can this get?” Resetting the Avengers roster with new faces just isn’t enough anymore, not when you’ve introduced a villain who can throw planets at people.
In the wake of the Mad Titan, rumors of Marvel toying with the idea of expanding its galactic boundaries beyond Thanos, Guardians of the Galaxy, and Captain Marvel (which introduces a whole new race of aliens called the Skrulls) via a film about The Eternals makes perfect sense. Now fans just have to make sense of the Eternals themselves.
The Celestials are ancient, powerful giants whose origin is unknown, but they are believed to have existed since the dawn of time - and they’ve already been established in the MCU. Remember Knowhere from Guardians of the Galaxy and Infinity War? It’s an entire mining colony built inside the severed head of a Celestial.
Created by Jack Kirby and introduced in the first Eternals comic from 1976, the Celestials are believed to be the oldest beings in the Marvel Universe. They created the Eternals, who in turn kickstarted the evolution of pretty much every other super-powered being in Marvel's wheelhouse.
The Celestials’ origin sounds like something out of classical myth. At the beginning of time, an omnipotent yet lonely intelligence called The First Firmament created the first life forms. These beings quickly broke into two factions, those loyal to the Firmament (called “Aspirants”) and those who were not.
These rebels created lower lifeforms that evolved and educated themselves to escape mindless enslavement. These were The Celestials.
During Earth’s stone age, the Celestials created three lifeforms: Eternals, Deviants, and humans.
The Eternals were powerful and beautiful beings. The Deviants usually had similar strengths, but one glaring weakness: they were often deformed or mutated or monstrous in appearance. The Deviants also tended to serve as antagonists - looking to conquer and destroy earth while the Eternals fought to defend it (or at least maintain sovereignty over it). If the Eternals enter the MCU looking to reclaim their position of power, the Deviants could either be a BIGGER bad or a “the enemy of my enemy is my friend” scenario for the existing movie heroes. Either way, it expands the universe in huge ways.
The Deviants are also arguably more important to the MCU than the Eternals for two reasons. Thanos carries the Deviant gene, which is why he’s purple and brutish in appearance (even though his parents are Eternals) and why he felt like an outcast among his own. Also, the Deviants planted the seed for mutation in humanity’s genetic code - so there’d be no X-Men without them.
Although they sound like replacements for some ancient pagan gods, the Eternals actually exist alongside real-life deities. Obviously, Norse gods exist in the Marvel Universe (see: Thor), as do Greek gods (see: Hercules), but the confusion comes from a pact the Eternals made with these other omnipotent beings.
Noticing similarities between themselves and the gods of Mount Olympus (the Eternal Zuras’s name sounds like Zeus, Azura sounds like Athena, etc.), the Eternals and the Olympian gods made a deal: the Eternals would act as the gods’ physical representatives on Earth.
This explains the heavy crossover between Marvel characters and real gods (in canon, the Eternal named Sprite is the inspiration for the character of Puck in William Shakespeare’s A Midsummer Night’s Dream).
However, the early humans couldn’t tell the difference between gods and Eternals (especially after the Eternal Ikaris adopted his name following the death of his son Icarus of Greek myth) and this annoyed the Olympians.