At first glance, Zack Snyder's Justice League isn't very different from Joss Whedon's theatrical cut in 2017 but as you watch it, the subtle (and some very glaring changes) become obvious and make all the difference, explaining why the film is getting such positive reviews. And perhaps the biggest change the story has undergone, apart from Ray Fisher's Cyborg, is how the villains have been portrayed. Steppenwolf is no longer a cringy leader only there to be beaten by the heroes and Darkseid is not just a barely whispered name. They have a bigger mission than uniting the Mother boxes- they are hunting for the evil, universe-altering weapon known as the Anti-Life Equation.
Darkseid nor the Anti-Life Equation never made it into Whedon's cut
Darkseid was, more or less, erased by Whedon when he came aboard to helm the Justice League. He even replaced him in the story, in which his forces invaded Earth and tried to unite the Mother Boxes, thousands of years ago. In the 2017 version, instead of him, it was Steppenwolf who came to Earth and was defeated by the combined forces of humans, Amazonians, Atlanteans, and the Gods. As Darkseid, his overlord, was barely mentioned once, Steppenwolf appeared as a wilful villain who just likes creating havoc and destroying worlds. And just like Darkseid, the bigger picture, i.e., the hunt for the Anti-Life Equation never made it to the film.
But Zack Snyder's Justice League restores it all
While the mission of uniting the three Mother Boxes to transform Earth and then take over the planet is what drives the story in this version too, they end up being mere MacGuffins. Even though annexing as many planets as possible is Darkseid's plan, the object he really desires, something that was supposed to stir the DCEU in a new direction, is the Anti-Life Equation that is hidden on a primitive planet.
So now, when Steppenwolf, desperate to win back Darseid's approval and return home, unites two of the three Mother Boxes, he gets a vision where he discovers that the Anti-Life Equation is written into the very surface of Earth. But what is the Anti-Life Equation? Before we dive into its dark comic history (scroll down), here is the definition Steppenwolf provides- it is the "ultimate weapon" that Darkseid has been searching for a long time, even before he rose to the throne of Apokolips. The Anti-Life Equation is "the key to controlling all life and all will throughout the multiverse." Eons ago, Darkseid found it hidden on a primitive planet but the inhabitants of the world fought back.
But as he was grievously injured in the fight, he had to return back to his planet. It is explained that once he returned, this world was lost to him and he couldn't find this primitive planet again. The only way that he couldn't have recognized that the planet was Earth is probably because the planet's appearance and location in the universe might have changed. Unlike this time, where Steppenwolf has more knowledge of the planet he has come to annex, Darkseid just brought his army to attack and fought against a mixed army of Amazonians, Atlanteans, Humans, Gods, and other alien races, which must have made it impossible for him to recognize it as, what is called, the "land of men."
Anti-Life Equation and what makes it a formidable weapon
In the comics too, finding and using the Anti-Life Equation to dominate the wills of all life forms and the universe as a whole is the ultimate goal of Darkseid. To define it precisely, the Anti-Life Equation is a powerful mathematical formula that holds the power to rewrite a person's will. Whoever masters this formula arms himself with the ability to control any and every living being's will- they will be mindless meat suits being puppeteered around by anyone who knows the full formula, having no freedom or ability to freely think. The ones who hear the formula are brainwashed into thinking that living is pointless, hope becomes a dead concept, and they are plunged into hopelessness.
As specified in the comics, the formula is- loneliness + alienation + fear + despair + self-worth ÷ mockery ÷ condemnation ÷ misunderstanding x guilt x shame x failure x judgment, n=y where y=hope and n=folly, love=lies, life=death, self=dark side.
In the comics, Darkseid found out about the Anti-Life Equation when he came in contact with the Martians. Their moral philosophy is based on the Life Equation, i.e., having free will. Darkseid postulated that if the Life Equation exists then its antithesis- getting the absolute power to dominate the very free will- must be present somewhere too.
Throughout the DC Comics' timeline, the Anti-Life Equation has undergone many changes, caused many catastrophic events, and was even a living entity for some time (though this version was never revisited again). But the equation majorly retained its core usage- the means to control someone's will, completely. At a point, Darkseid even managed to find the formula and used it to subjugate billions of people, including superheroes like Wonder Woman and villains, on Earth to his will, making them his slaves. Ultimately, Wonder Woman broke free from the hold of the equation and defeated Darkseid through the Lasso of Truth, thus ending his reign on Earth.
While it is highly unlikely that a sequel to Zack Snyder's Justice League will ever be greenlit, going by the massively positive reactions the film has received, maybe just maybe, Ava DuVernay's New Gods movie, which is set to reintroduce Darkseid, DeSaad, etc, will properly explore the existence and dire implications of the Anti-Life Equation.
The Snyder's Cut's ending teases a world where Darkseid has actually attained the formula and used it on Earth, enslaving even Superman. Maybe we will still get to see Darkseid employing the evil equation, who knows...well, Warner Bros. does, but still, fingers crossed! Zack Snyder's Justice League is currently streaming on HBO Max.