Thanos isn’t an Eternal, but he might be related to them.
The Marvel Cinematic Universe has delivered three distinct storytelling phases. Phase 1 was an introduction to Earth’s mightiest heroes up until their team-up in The Avengers. Phase 2 showed the aftermath of The Avengers through Age of Ultron and ending with the small-scale debut of Ant-Man. Phase 3 is the current phase, which chronicled the dissolution of the Avengers in Captain America: Civil War and will end with after Spider-Man: Far From Home takes place post-Endgame.
As for what Phase 4 will be about? There are only a few hints. A Black Widow film is certainly in the lineup, but that will likely be a prequel to the events of the current MCU timeline. Casting announcements also revealed plans for a Shang-Chi movie.
Most saliently, Angelina Jolie and Kumail Nanjiani are attached to star in The Eternals, which looks to be the MCU’s next attempt at introducing a new team of heroes to its massive audience. The Eternals are less known to non–comic reading audiences so there’s plenty of room for interpretation, but for those who want a primer on who and what the next big batch of Marvel characters are, let’s do a rundown.
There are various versions and reboots of the Eternals in their 40+ years of history at Marvel, but the general idea is that long, long ago, the godlike Celestials came to Earth and used their science-magic to create two divergent species based on the DNA of early humans. These species were Eternals and Deviants.
(If the Celestials sound familiar, one of them has already turned up in the MCU. Ego the Living Planet, A.K.A Star-Lord’s dad, was a Celestial. That makes Peter Quill half-Celestial, even if his dad kicked the bucket in Guardians of the Galaxy 2).
Eternals are human-shaped and nearly immortal in that they do not get sick and are immune to toxins that would kill an average human. Their bodies are also extremely durable and they are nearly impossible to destroy. By default they possess the powers of flight, telekinesis, super strength, mind reading, force field generation, and more depending on the Eternal, and they can enter a hive-mind state with each other that collates their powers into an amplified being called the Uni-Mind.
The Eternals are usually depicted as warring with the Deviants to protect the human race. Some even settled in the mountains of Ancient Greece and founded an Eternals city called Olympia, thus becoming the real-life basis for the Greek pantheon.
Deviants got the substantially shittier end of the experiment stick and are physically malformed and have lesser powers compared to Eternals. Thanos was a Deviant in the comics, and while his heritage has not been explained in the movies so far his presence may be used as a later example of their presence in the MCU.
Also, comic lore states that the experiments behind the Eternals and Deviants bled into the human population and led to the series of human mutations that give mutant characters like the X-Men their powers.In that way, the Eternals entering the MCU may serve as a backdoor way for Marvel Studios to introduce the X-Men and other mutant superheroes, now that its parent company Disney owns the rights to the mutant characters previously owned by 20th Century Fox.
The only two named Eternals who are for sure turning up in the Phase 4 movie are Sersi and Ikaris. Megastar Angelina Jolie is signed on to star as Sersi, an Eternal who is considered unique among her kind because she prefers to spend her immortal life among humans. She is a particularly powerful Eternal and was even an Avenger at one point in the comics— partly because she had a crush on Captain America. Relatable.
Ikaris has not yet been cast, but he’s another unique Eternal. He is skilled in the projection of energy from his eyes and hands and also has a strong interest in humans — even if his name is a nod to his history among mortals on earth, since the comics place him as the father of the mythological human Icarus, who flew too close to the sun on mechanical wings.
Again, there’s a lot of history about the Eternals and Deviants in Marvel Comics and there’s no saying which bits of their backstory the MCU will use for its interpretation, but knowing that they’re immortal superhumans with powers beyond anyone else in the MCU should insinuate that the power levels in Phase 4 are about to skyrocket. Whoever remains after Endgame will have to content with an entirely new class of hero…and a potentially worse category of villains.