WARNING: The following article contains spoilers for Zack Snyder's Justice League, directed by Zack Snyder, streaming now on HBO Max.
One of the most heavily discussed elements of Zack Snyder's Justice League has been the sequence in the film's epilogue set in what Snyder refers to as the Knightmare, the possible post-apocalyptic future timeline Bruce Wayne/Batman (Ben Affleck) first glimpsed during Batman v Superman: Dawn of Justice.
The Knightmare scenes depict a time where Earth has been conquered by the forces of Apokolips, led by a corrupted Superman (Henry Cavill), presumably on behalf of Darkseid (Ray Porter), although the alien warlord doesn't actually appear in either sequence. The world of the Knightmare is very much a unique creation of Snyder's and his fellow filmmakers' but it does bear similarities to a handful of past DC stories from other media, suggesting that those tales were influences on the films.
Injustice: Gods Among Us
Injustice: Gods Among Us originated as a fighting video game set in a world where Superman has led a group of DC heroes in turning the Earth into a totalitarian state, while Batman leads a resistance. The game spawned both a sequel and an extensive library of comics set in its version of the DC Universe by creators like Tom Taylor, Brian Buccellato, Xermanico, Tom Derenick, Mike Miller, Bruno Redondo, and Daniel Sampere.
There are several similarities between the world of Injustice and the Knightmare sequence, particularly regarding how the corruption of the Knightmare Superman is a result of Lois Lane's (Amy Adams) death. Snyder has elaborated on how his plan for future films would have seen Superman succumbing to the Anti-Life Equation after Lois died. Injustice Superman's turn was motivated by his Lois dying as a result of one of the Joker's schemes. However, he ultimately became a tyrant of his own free will, unlike Cavill's version. In Injustice, Lois was pregnant at the time of her death and while Zack Snyder's Justice League also showed her as such it's unclear if she would have been at the time of her death in this timeline.
Final Crisis
One of DC's darkest events, Grant Morrison and J.G. Jones' Final Crisis saw Darkseid conquer Earth after falling through time. When the Anti-Life Equation is released on the Internet most of Earth's citizens and heroes become his servants, while the few remaining freedom fighters also have to battle the Dark Monitor Mandrakk in a conflict that redefined the DC multiverse.
Unlike in the Knightmare sequence, Superman remained heroic throughout the event, although his evil doppelgängers Overman and Ultraman did appear, with the latter in particular posing a grave threat. Lois is also severely injured in Final Crisis, though Clark is ultimately able to save her. Snyder's plans for Justice League sequels also had several elements in common with Final Crisis, including the apparent death of Batman.
JLA: Rock of Ages
An earlier Morrison story, with Howard Porter, put the League through a tough battle with Darkseid. "Rock of Ages" was a story arc that ran from JLA #10-15. The present day Justice League are in the midst of battling Lex Luthor and the new Injustice Gang when the New God Metron comes to them with a warning: If they defeat Luthor it will allow Darkseid to conquer the Earth at a future time.
Aquaman, Wally West Flash, and Kyle Rayner Green Lantern are teleported to this future, which bears a resemblance to the Knightmare, especially with Batman as one of the few surviving heroes. Luthor's involvement in the story is also similar to Snyder's films, in which Jesse Eisenberg's version of the character contacted Steppenwolf (Ciarán Hinds) and it was his plan that resulted in Superman's death, which in turn awakened the Mother Boxes, which sensed an opportunity for Apokolips to invade.
Superman: The Dark Side
The idea of a Darkseid-controlled Superman was famously presented in this limited series from 1998. An Elseworlds tale, Superman: The Dark Side, by John Francis Moore with pencils by Kieron Dwyer, depicts an alternate reality in which Kal-El's rocket landed on Apokolips, rather than Earth, following the destruction of Krypton. Kal is raised by Darkseid and serves as his soldier for years before relenting, after which he helps free New Genesis from his adoptive father's control.
Like Injustice, The Dark Side differs from the Knightmare in that Superman willingly served Darkseid rather than being corrupted by the Anti-Life Equation. Cavill's Superman would have eventually also broken free of Darkseid's control, although not until much later in Snyder's planned DC saga.