WARNING: The following article contains spoilers for Eternals #2 by Kieron Gillen, Esad Ribic, Matthew Wilson, and VC's Clayton Cowles, on sale now.
While Thanos was killed at the conclusion of Avengers: Endgame, it didn't stop him from leaving a titanic impression on the Marvel Cinematic Universe. With The Eternals film scheduled to be released later this year, Marvel will introduce a group of massively powerful cosmic beings who have the power to shape, protect, and destroy the entire MCU.
While the Eternals were nowhere to be found during Thanos's invasion of Earth, they both share an incredibly intimate connection that could possibly be explored in the upcoming film.
The Eternals' connection to Thanos was spelled out explicitly in Kieron Gillen and Esad Ribic's Eternals #2. Originally, the Eternals were created by the Celestial Gammenon the Gatherer to oversee the development of humanity in the early days of the world.The Eternals can tap into pure cosmic energy which gives them their angelic appearances and reality-shattering powers. However, the Celestials also created the Deviants, a group of vastly different individuals with more unstable genomes that left room for genetic mutations of all sorts. The Eternals were given the duty of protecting Earth from the Deviants, which eventually lead to a civil war between different factions of Eternals.
Ultimately, one Eternal, A'Lars, was allowed to found a colony on Saturn's Moon, Titan. There, he found Sui-San, the last surviving follower of an exiled Eternal, and the two successfully formed a colony. This group of Eternals evolved into an offshoot of the main Eternals line that was similar, but not identical, to Earth's Eternals.
Together, A'Lars, also called Mentor, and Sui-San genetically engineered several children and had a few biological children, including the Avengers' Starfox. However, one of their children was Thanos, who was a Deviant of Titan.
While knowledge of the Titan Schism was largely wiped from the minds of the Eternals in an effort to preserve peace, the resulting experiment was largely a success, barring the creation of Thanos. However, this effer did lead to the creation of Thanos, who would become one of the greatest villains in the entire universe, famously going on to kill billions with the power of the Infinity Gauntlet.
Since Thanos is both a Deviant and an Eternal of Titan, he's a few steps too far removed from Marvel's main Eternals to be considered one of them. However, he still has a genetic connection to the Eternals and stands as the darkest part of their legacy on the Marvel Universe.