One of the big elements that Zack Snyder's Justice League sets up and explores is the Multiverse, a concept that future DC films will utilize down the line. When Justice League was tampered with back in 2017 by Warner Bros. and Joss Whedon, a lot of characters, concepts, and plots were either retooled or removed completely. Multiple supporting characters like Iris West, Ryan Choi, Elinore Stone, and Desaad were nowhere to be seen in the theatrical cut. The biggest change to the villains, besides Steppenwolf's visual appearance, was the complete erasure of the DC Comics baddie Darkseid.
But with Zack Snyder's Justice League, everything that was changed or removed from the theatrical cut was restored in the 4-hour cut. With Ray Porter's Darkseid now also servicing as the ultimate big bad of this story, another removed concept came with his introduction: the Multiverse. Even though the Snyder Cut was prominently about the foundation of the iconic DC team, it also started to build up the cinematic incarnation of the Multiverse that is a big part of the comics. While it will be a bigger plot in The Flash movie that involves both time-travel and multiverse exploring, the Zack Snyder's Justice League does give it its focus, primarily through Darkseid.
While the Multiverse has had had a big presence in live-action DC properties, primarily through the Arrowverse, Zack Snyder's Justice League becomes the biggest DC movie to go further into it. Even though alternative Earths are never physically visited, like an Earth-2 or Earth-89, they do establish a lot with Darkseid's overall goal in wanting to rule the Multiverse.
Darkseid Is Conquering The DCEU Multiverse In The Snyder Cut
While he doesn't have many scenes in Zack Snyder's Justice League, Darkseid's motives are made clear as day with the handful of moments where he does pop up. It's explained that Darkseid has been conquering world after world for complete domination. Darkseid even tells Steppenwolf that he has turned "100,000 worlds to dust", implying that one-tenth of a million alternative planets have been either destroyed and conquered by the Apokolips lord. While Darkseid has been focused on taking one world after another, it has also been in the midst of him looking to claim the Anti-Life Equation.
The formula that kills all free will in anything and anyone in the whole universe is Darkseid's key to absolute control. The Multiverse gets further alluded to when Desaad states that Steppenwolf owns Darkseid another 50,000 worlds before being able to redeem himself. They never specify too much on how Darkseid and the forces of Apokolips are traveling between the Multiverse. Because as much as he could have taken over planets in this universe, having done it to 100,000 others means that Darkseid has traveled into alternative universes.
The Flash's Re-written Timeline In Justice League
The Multiverse is not only about alternative worlds, but also alternative realities that are created through time-travel. In Zack Snyder's Justice League, the team actually does get taken out when the Mother Boxes synchronized in The Unity and were going to cleanse the Earth. But the hero that was able to stop it, before Darkseid could come to their world and collect the Anti-Life Equation, was Flash. Barry taps into the Speed Force and pushes himself to start rewind time itself in order to give the Justice League another chance. As Barry is running, he gives the team, but primarily Cyborg, enough time to give him the charge to help him stop the Mother Boxes. This causes a new reality to be born when Barry rewrites the timeline where Darkseid initially won.
Had Barry not used his speed to rewind the events, Earth would have fallen to Apokolips and Darkseid would finally have the Equation. Instead, two realities exist from that specific moment, with the heroes living in the one where they disabled the Mother Boxes, but also killed Steppenwolf. Despite being told that there were no protectors in this world, Darkseid learns that a new generation of heroes has come together with the intention to stop him. But with Barry having gone back in time before The Unity wiped out everything living on Earth, it may have paved the way for another reality that Snyder put into place back in 2016.
Justice League's Knightmare Future Timeline
Zack Snyder's Justice League ended with a big cliffhanger that went back to another big plot that was first set up in Batman v Superman: Dawn of Justice. At some point in the future, Darkseid gets what he wants by conquering Earth while also wielding the power of the Anti-Life Equation. Not only are Wonder Woman and Aquaman dead, but Superman has succumbed to the Anti-Life Equation after Darkseid murdered Lois Lane. Together, they take over the planet and become another fallen world in Darkseid's massive track record. This is what forces Batman to align himself with the remaining heroes of the Justice League, but also a few long-time villains of his, including Joker and Deathstroke.
This future is known as the Knightmare timeline which is what Barry tried to tell Bruce in 2016 when he showed up in the Batcave to warn Batman about the future. Despite having changed the past to a degree, the Knightmare reality still happens in Snyder's overall story. If Zack Snyder's Justice League 2 were to happen, it would completely be set in the Knightmare world. Had Barry not rewritten the climax, the Knightmare story would have essentially been prevented from happening. But since he did rewind time and stop Unity's synchronization as well as have Steppenwolf be killed, this is partially what makes that reality possible.
Because of the creation of the Knightmare, that's why the remaining forces had to build the cosmic treadmill so The Flash could go back in time to warn their past counterparts. Barry does, unfortunately, show up too early in Batman v Superman but it was a warning to at least get Batman aware of what's coming for them in a few years. If Zack Snyder's Justice League 2 and even Zack Snyder's Justice League 3 happens, it would allow the director to continue fleshing out his version of the Multiverse.