The following contains spoilers for Black Adam, now showing in theaters.
One of the most intriguing aspects of Black Adam is how Dwayne Johnson's Teth-Adam seems so unstoppable. It's why the Justice Society comes to subjugate him in Kahndaq, scared a weapon of mass destruction, if unchecked, could wreak havoc all over the world. And as expected, Adam pulls no punches, mercilessly slaughtering Intergang members who encroach on his territory while reminding the heroes all invaders, including foreigners like them, will be punished.
Black Adam eventually does get one challenge, though, in the form of the demonic Sabbac. But again, Adam destroys that threat, coming off as the only person in the DC Extended Universe who might be able to kill Superman. However, the film also introduces a weakness for the antihero, but for some reason, this Achilles' heel is dropped from the plot early on.
Black Adam Is Vulnerable to Eternium
When Adam is released from his cave by Adrianna, a former university professor, he decimates an Intergang legion seeking to find the crown of Sabbac. He's got flight and super-strength like the Man of Steel, not to mention he's bulletproof. In addition, he possesses the speed of The Flash, making it easy for him to destroy the army in mere seconds.
It paints him as a veritable god, but when he catches a rocket, he sees "eternium" marked on it. It blows up, with the energy leaving a wound in his stomach, knocking him out. Adrianna's crew then takes him back home, where he heals and cauterizes the cuts via his lightning. Adam shakes it off and goes about the movie, with the eternium not playing a major role. He does get cut from the crown -- which has the metal in it -- later on, but even when the proactive Hawkman sees this as a flaw, he doesn't think about possibly using the precious metal.
Eternium Is Magical Kryptonite in the DCEU
As for why eternium's so lethal, the film reveals early on it's a conduit for magical energy, which is why Intergang has harnessed it as a fuel source. Centuries prior, Kahndaq's evil king made people mine for it so he could create the crown and bring forth Sabbac. The Black Adam prequel comics laid the groundwork for this, revealing the metal comes from the Rock of Eternity. It's why, when Adriana enters the tomb where Adam's imprisoned in the opening act, her eternium chain glows and reacts to his presence.
Now, this story, including how the metal's traits harm Adam, would have been a great story to explore, especially considering Hawkman and Co. needed an equalizer. Waller had secret files on Kahndaq's past, so information on weaponizing it should have been important. But not even Adrianna addresses the eternium, despite knowing so much history and how the metal's linked to the mystical dilemma at hand.
Sadly, the movie doesn't give Sabbac use of the crown or have his horns made from eternium, which would have been perfect to kill Adam. It's like Kryptonite getting discovered in a Superman story, only to be ignored. That said, maybe Hawkman will make a mace from the metal in future films, keeping it the same way Ben Affleck's Batman had Kryptonite for the Man of Steel. But as it stands, the eternium is such a powerful arc to leave on ice, especially after it's shaped as a game-changer to Adam's reign of unforgiving retribution.
See how eternium is discarded from the plot in Black Adam, now playing in theaters.