The Knightmare Was Zack Snyder's DC Movie Secret Weapon
The Knightmare future was a major plot point in Zack Snyder's DC movies, and it was also secretly the biggest asset of his planned 5-movie arc.
The Knightmare future was of Zack Snyder's DC movies was the secret weapon of the DCEU. Thus far only seen in relatively brief segments in Batman v Superman: Dawn of Justice and Zack Snyder's Justice League, it was intended to be a defining, centralized apocalyptic event in Snyder's planned 5-movie arc. After some behind-the-scenes alterations to Justice League 2 (the original plans were which were seen on a collection of whiteboards at a Texas AT&T exhibit in early 2021, along with those of Justice League 3), the fourth film in Snyder's arc effectively became a Knightmare-centered story, per Snyder's own explanation of the whiteboards.
The basic idea of the Knightmare future involved Darkseid conquering the Earth, with Superman brought under his control via the Anti-Life Equation, while Batman, the surviving members of the Justice League, and a few other unlikely allies devising a plan to re-write history by altering the timeline. The Knightmare was essentially Zack Snyder's spin on the story of the Injustice comics and video games, and was meant to be a pivotal event in the story. While that much has been clear long before the Snyder Cut's release, the Knightmare was also a Trojan Horse in several other aspects.
Because nothing like the Knightmare had ever been done before in a DC movie, Justice League 2 would've been a superhero film that would essentially have nothing else to compare it to. However, apart from the basic post-apocalyptic setting and the dark Superman premise of Brightburn being turned up to 11, the Knightmare had a few other hidden assets that made it the DCEU's silver bullet in multiple ways. Here's how the Knightmare future of Zack Snyder's 5-movie arc was the secret weapon of the Superman-centered story his films were telling.
In creating a scenario where Darkseid has conquered Earth, the Knightmare future presented a whole new canvas to use the Justice League and many DC villains in ways they never have been on film before. Superman would be turned into an Injustice-level antagonist, though without actually turning evil, since he was under Darkseid's control via the Anti-Life Equation found on Earth. Batman would also not only form a new Knightmare version of the Justice League, but the situation would force him to join forces with surviving villains like Deathstroke and the Joker (both of whom had, by this point, killed people close to Bruce Wayne). Nearly every major hero and villain involved in the Knightmare would also receive a big costume makeover, while the setting itself was to be an apocalyptic future right out of Mad Max: Fury Road.
One big added benefit of this was that it also presented Warner Bros. and DC with a merchadising and spin-off goldmine. With the studio and its licensees working backwards from wanting to sell toys and Halloween costumes being how 1997's Batman & Robin came together, Snyder's Knightmare future had all of that baked right into the story. Snyder's comedic segment on The Late Show with Stephen Colbert highlighted this in Colbert's comment on the potential for "spin-offs, animated specials, theme park rides" from what the segment set up in the "Extended Late Show SnyderVerse". The Knightmare itself offered similar potential for vast expansion into comics, video games, one-off series devoted to one or two characters, and countless other possibilities. A "DC Knightmare Land" or Flash Cosmic Treadmill theme park attraction at Six Flags, for example, plays right off of Colbert's comment on the Knightmare as a gift that could keep on giving. Additionally, the Knightmare also created a double-layered story format for Snyder's 5-movie arc.
With the Knightmare having been shown in Batman v Superman and Zack Snyder's Justice League in the form of flashes into a post-apocalyptic world, Snyder was essentially telling two stories at once through alternate timelines (time-travel being an element of Snyder's films that WB also reportedly balked at). With this first three movies showing the present day for the Justice League, the Knightmare was being set up as a future that Batman, Cyborg, and the audience have had just a peek into before the franchise would finally take a full plunge into it in Justice League 2. At that point, the mission for the surviving members of the League was about undoing that future by sending The Flash back into the past on the Cosmic Treadmill.
In a nutshell, Snyder's arc had the present and future Justice League as two separate stories that would eventually converge into one in Justice League 2. The Knightmare set up a distinct story of the League's defeat and a world ruled by Darkseid with Superman as his right-hand man, with Justice League 3 returning everything to the pre-Knightmare status quo to set up the League's final battle with Apokolips under Superman's leadership. With the Knightmare, Snyder was basically pulling a Back to the Future Part II, setting up an alternate, hellish world as an integral element of his story, but also a parallel one to the Justice League's linear trajectory that would be re-established after the Knightmare timeline was averted.
Snyder's planned 5-part arc has been described as Superman-centric, since the franchise began with his DCEU origin in Man of Steel, but it was still an ensemble story that left none of the heroes shortchanged. One of the more subtle aspects of the Knightmare is that it positions Batman and Superman as parallel figures of equal significance, with the respective stories both being individualized and complementing each other. The Knightmare shows Batman occupying the same leadership role he took on in forming the Justice League. Batman v Superman showed Bruce Wayne as a broken man pulled back from the brink by Superman's sacrifice and becoming a hero again. In the Knightmare, the Man of Steel has fallen victim to the same fate, with Batman similarly pulling him back from what he's been turned into.
Superman's alignment with Apokolips in the Knightmare was triggered by Darkseid's killing of Lois Lane, similar to Batman's earlier fall from grace after Robin's death, with Darkseid using the Anti-Life Equation to bring the now vulnerable Superman under his control. Batman's plan to rewrite history would free Superman of Darkseid's control in the re-set timeline, and lead right into the conclusion of both of their stories in Justice League 3. The Man of Steel would become Superman fully realized, leading the League and the armies of mankind in the final war against the Apokoliptian invasion, while Batman would make the ultimate sacrifice to defeat Darkseid once and for all. The Knightmare would've been the three-quarters point for both of their stories, with their roles in it being major elements of where each was on his journey and individually setting up how their arcs were to conclude.
Though the Snyder Cut has finally been released, the role of the Knightmare remains another "What if?" in the unrealized two films of Snyder's planned story, due to Warner Bros. stance that Zack Snyder's Justice League is the end of the line. Of course, with ongoing #RestoreTheSnyderVerse calls having reached mainstream proportions, to the point of even surpassing 1 million tweets in a day, what Zack Snyder had planned with the Knightmare has very little chance of slipping out of the public consciousness. Whether Snyder's fans can make history twice is still up in the air, but the importance of the Knightmare to Snyder's plans is not only clear, the hidden assets it brought with it made it into most unexpected tool in the franchise's proverbial utility belt.