The Eternals Have Emerged as Marvel's Next Cosmic Supervillains
Warning: Contains spoilers for Eternals: The Heretic
Marvel Comics has been expanding the mythos and story of the Eternals a lot recently, mostly in the pages of the Eternals series, written by Kieron Gillen with art by Esad Ribic. A new miniseries, Eternals: The Heretic (by Kieron Gillen, Ryan Bodenheim, Edgar Salazar, and Chris O'Halloran), further explores the Eternals' past and, by doing so, sets up the stage for the reclusive race of superpowered beings to become the next cosmic threat for the Marvel Universe.
The Eternals were created by the inscrutable gods known as Celestials eons ago, together with the Deviants, in order to protect and nurture the development of Earth and its creatures. The Eternals' job was to correct "excess deviation", something that they interpreted as getting rid of the Deviants when their unstable genetic code turned one of them into a dangerous monster, something referred to as "the curse". While the Eternals are unchanging, immortals, and fixed in numbers, the Deviants are mutating and reproducing, so their respective "tasks" continued for millennia, always guided by "the Three Principles" inscribed into each one of them by their Celestial gods. One Eternal, however, decided that he understood the principles better than anyone else, and saw a better way to fulfill the duty of his species.
Uranos is one of the Three Patriarchs, incredibly powerful Eternals who preside over a "dynasty" of their descendants. Uranos believed that "any deviation is excess deviation", so the Eternals should erase any trace of the Deviants species, not just of the few who got taken by the curse. In this way, the Eternals would serve their Celestial purpose in the best way possible, and then be free to pursue their destiny. Uranos' brothers, Kronos and Oceanus, saw his proposal as genocide and opposed him, starting a war that ended with Uranos' defeat and imprisonment. Before that, however, Uranos shared his "theses on the Principles", and by looking at them it is clear that, if he ever got his way, the Eternals would become the greatest force of destruction that the universe ever witnessed.
Besides the Eternals' mission to "correct excess deviation", Uranos had dangerous interpretations of the other two Principles, "protect the Machine" and "protect the Celestials". According to him, the Machine has to be identified simply as the infrastructure of planet Earth and the Eternals, and all existence outside of it is a possible threat. In this impossible xenophobic view, the Eternals should make the planet mobile, and traverse the universe purging everything outside of it. There is a conflict between the Principles and the existence of anything outside the Machine, meaning that the Eternals are at war with the universe. The Celestials should be protected, but the best way of doing that, according to Uranos, would be imprisoning them and harnessing their power.
The idea of the Eternals making Earth essentially a planet-sized warship traveling the universe and using the Celestials' power to purge every other life form is indeed a scary one, and Uranos' imprisonment doesn't mean that it will not still happen. Thanos recently became Eternal Prime, the leader of the Eternals, and he learned everything about Uranos and his past. The Mad Titan also acquired the key to the weapons of mass destruction developed by Uranos that could destroy Earth easily, but what would happen if he decided instead to follow his grandfather's original plan? Purging all life in the galaxy would be very much in line with Thanos' nihilistic views (and his previous actions). If he decided to make Uranos' theses his own, Thanos could turn the Eternals into the greatest threat that the Marvel Universe has ever seen.