Warning: the following contains spoilers for Justice League.
Justice League brings Jack Kirby's Fourth World of the DC Universe to the big screen in a big way, from the Apokaliptan Steppenwolf who wants to join the ranks of the "New Gods" to Parademons and Boom Tubes, but the number one addition to the DC Extended Universe's lore is the film's MacGuffins: Mother Boxes.
In the comics, Mother Boxes are a sort of one-size-fits-all mystical alien artificially intelligent technology that can basically do anything from interstellar transportation, communications, prophecy, advanced healing, you name it. While the movie boxes are a little different, making tweaks to the lore in some instances and leaving other aspects unexplained, they don't stray too far from what we know about them from the comics.
The very first appearance of a Mother Box in the DCEU is in Batman v Superman: Dawn of Justice during the video clip stolen from Lex Luthor's server featuring Silas Stone turning his son Victor into the Cyborg. While Victor was clearly brutally injured and missing his limbs, the Mother Box was able to keep him alive, integrating itself into his body, and possibly even trying to take over his mind as we see in Justice League where he hints exactly that and even struggles with his enhancements from time to time, including taking a shot at a newly resurrected Superman, triggering a battle.
The next appearance is also in Batman v Superman (at least the Ultimate Edition) where Lex uses the Kryptonian Scout Ship to communicate with a figure - who we now know is Steppenwolf - who is holding 3 Mother Boxes, followed by a scene where insane Lex Luthor tells Batman "But the bells have already been rung and they've heard it. Out in the dark. Among the stars. Ding dong, the god is dead" before screaming "The bells cannot be unrung! He's hungry. He's found us. And he's coming!" clearly setting up for Steppenwolf's arrival to claim the boxes.
While Batman v Superman teased Steppenwolf's arrival and the Mother Boxes, Justice League takes a much deeper dive, going back five thousand years to when Steppenwolf first tried to conquer Earth. During this flashback, as told by Diana, Steppenwolf arrives on Earth with an Apokaliptan army, bringing out the Mother Boxes to perform an event called "the Unity" that would merge the boxes together and cause the entire planet to be transformed into the nightmarish hellscape of his homeworld - Apokalips. While he doesn't need the cooperation of the people Earth to do this, he desires to defeat its inhabitants first and make them kneel before him before he performs the Unity and terraforms the planet.
Fortunately, an alliance of Atlanteans, Amazons, and all races of men unite to fight him off, along with the aid of a Green Lantern and a few of the Old Gods, including Zeus and Ares, driving him to retreat without his precious Mother Boxes. Unable to destroy the boxes, but desiring to prevent the Unity from happening again, the 3 boxes are divided up between the three kingdoms.
Steppenwolf would stay away for five thousand years, but after the death of Superman, fear begins to overtake the Earth, causing the Mother boxes to summon the Apokaliptan general once again, only this time he's not messing around. Not only are the Earth's people divided, but he's planning on skipping the whole conquering part to perform the Unity as soon as he obtains all three boxes once again.
While he's able to quickly retrieve the Atlantean and Amazonian boxes, Cyborg is protecting the final box - the one that turned him into what he is. Now that the League is in possession of the box Bruce gets the idea that it could be used to revive Superman. Mother Boxes are powerful enough to transform a planet, but if Superman is stronger than a planet, and his cells can't be deteriorated, the Mother Box might be able to supercharge those cells and bring him back.
The plan is a success, but Superman isn't very friendly when he comes back, so while he struggles with the League, Steppenwolf is able to Boom Tube in and abscond with the last Mother Box, taking it to Russia to begin the Unity once again, where we see exactly what would have happened during his first attack had he not been driven off. Alien terrain begins springing forth from the combined boxes and clouds form above, turning the skies red - a classic sign of a "crisis" event in the comics.
Having been made from a Mother Box, Cyborg has the technical ability and the know-how to stop the Unity, and it turns out it's a fairly straightforward process - just separate the boxes with brute force. To be fair, he does interface with the unifying boxes for some time prior, so it's possible he did something more technical the movie doesn't explain, but ultimately it comes down to Cyborg enlisting the aid of Superman to pull them apart, stopping the Unity and saving the world.
Once the boxes are separated, there's no explanation for where they go or how they're dealt with. They could have vanished, or maybe the heroes returned them to their proper guardians, perhaps keeping mankind's Mother Box in the new Hall of Justice - AKA the old Wayne Manor. It'd doubtful the boxes will be a central plot point for Justice League 2 (if there is a Justice League 2), but you never know. If Darkseid ever shows up in the DCEU, there will likely be some connection to Jack Kirby's fourth world, the Mother Boxes, as that's pretty well baked into Darkseid's orgin and most stories where he shows up.
It's clear there's a lot of Zack Snyder's lore and worldbuilding was excised from Joss Whedon's final cut of the movie, so it's definitely possible Zack Snyder gave more history and explanation for the boxes, but it's only possible to fit so much into a two hour runtime, so the ultimate product is more of a crash course of lite DC mythology. Thanks to the movie's abysmal box office returns, it seems unlikely that much of the plot or characters of Justice League, other than Wonder Woman and Batman, will be explored in the near future, but if Warner Bros. can get the DCEU back on its feet, hopefully they return to Jack Kirby's Fourth World for some more crazy sci-fi mythology.
- Justice League (2017) release date: Nov 17, 2017
- Aquaman (2018) release date: Dec 21, 2018
- Shazam! (2019) release date: Apr 05, 2019
- Wonder Woman 2 (2019) release date: Nov 01, 2019