WARNING: The following article contains spoilers for Eternals: Celestia #1, on sale now from Marvel Comics.
A recollection of a past battle in Eternals: Celestia #1 (by by Kieron Gillen, Kei Zama, John Livesay, Matthew Wilson, and VC's Clayton Cowles) revealed that the Eternals have a crippling weakness that they share with Superman: a vulnerability to magic.
While not unprecedented in comics, the Eternals have always been somewhat above it all, and for them to have such a glaring weakness paints them as much more vulnerable than expected. This new revelation could have consequences for their MCU debut as well, providing a weak spot for their enemies to exploit when they appear on the big screen.
While narrating the fight between Ajak and the Avengers of 1,000,000 B.C.E. it listed true magic as the Eternals' "weakest spot" when the Eternal went up against the Marvel Universe's first sorcerer supreme. But this throwaway comment reveals that despite their vast capabilities, the Eternals do have a major weakness that quite a number of heroes and villains can exploit.
The reason why Superman is vulnerable to magic is because he, at the end of the day, is still a mortal being. For all his strength, the Man of Steel is a man whose powers come from a biological reaction to the yellow sun he lives under. In short, his powers are entirely natural. And magic affects the natural, transforming it into something extraordinary. Thus Superman is vulnerable to magic because it affects him like it does everyone else. The same principle must apply to the Eternals, perhaps even more so considering the nature of their existence.
The Eternals are beings of science. They were created by space gods and their entire cycle of death and rebirth is ensured by a machine. Their technology is advanced and while it seems like magic it is by definition science based. While this makes them powerful, no Eternal has truly delved into the mystic arts. As a result, they are vulnerable to it because they have not made any effort to understand it, perhaps deeming it beneath them or just not able to comprehend it.
The Eternals are about to make their MCU debut, picking up where heroes like Iron Man and Captain America left off. But with this new weakness exposed it could paint a target on their backs. As new heroes are appearing in the MCU, magic is becoming more and more prevalent. Doctor Strange ushered it onto the center stage and it isn't going away any time soon, ensuring that even the god-like beings will face major challenges upon entering the cinematic universe.