Zack Synder's Justice League did a lot of things to correct the theatrical version of the film, and one of these things was giving fans a new look at the Knightmare future. This future is a timeline in which Darkseid manages to control Superman through the Anti-Life Equation and destroys the earth.
This might be a bit confusing to DCEU fans, but there are several key things to be aware of in the Knightmare future that help to paint a fuller picture of what it would have looked like.
Darkseid has managed to take control of Superman in this future and is part of the reason that earth has turned into what it has. Through the Anti-Life Equation, Darkseid controls Superman, and, if there's anything that fans know about Superman, it's that he's one tough person to oppose. Any who try to go against his will are swiftly defeated. However, there is a reason that he is this way. He lost the love of his life, which made him vulnerable to Darkseid.
In this version of the future, Lois Lane died, which is the reason that Superman fell vulnerable to corruption. When he doesn't have Lois Lane, he is a very dangerous person. There's more to this, though. Lois Lane didn't just die, but her death could have been avoided had Batman sprung into action.
With that in mind, it stands to reason that Batman is a major reason that this future has become what it has, as he was able to stop it at one point in time.
It's currently unknown exactly what happened to Lois Lane, but she somehow died at the hands of Darkseid and Batman could have somehow stopped this. However, she was one of many characters to die in this future.
Just like Lois Lane, it isn't exactly clear what happened to Harley Quinn. She was killed and died in Batman's arms, but it's unclear whose side she was on during the war with Darkseid. However, Batman told Joker that her last request was that Joker die slowly. Was he the cause of her death, or had things simply deteriorated between the two as time went on? It's something we may never know.
The war with Darkseid clearly caused an irreparable amount of carnage. One of the victims of this is the King of Atlantis himself, Aquaman. He likely died in an attempt to stop Superman, as there are very few others who can stand up to Arthur. Whoever did it didn't stop there, though, as another member of the Justice League also died.
The comics have shown that if there are any who equal Superman in power, it's Wonder Woman. She's defeated him in battle a few times, though he's managed to take her down as well in the past. Clearly, he won this battle in the Knightmare future.
The details surrounding Wonder Woman's death in the Knightmare future are hazy, as are many others. However, if there's anything fans know, it's that Diana never stops fighting for justice. It's likely that she refused to give in until the very last second.
It was shown in Batman V. Superman that Flash traveled back in time to try and prevent this future from happening. While he overshot his time travel and ended up coming to Bruce before the Justice League was formed, the warning wasn't ignored. Bruce took note of it and did form the League. It's possible that because of this, the Knightmare future isn't ever realized. Or, perhaps, it was in vain.
Batman is absolutely out of allies by the time that viewers get to see the Knightmare future. He's traveling with some interesting people, including Joker himself. As his greatest villain, fans know that Joker and Batman working together means that there are simply no other options. However, his group has a few other additions that are a bit unexpected as well.
While Flash and Cyborg are still alive in this timeline, the group that Batman is working with is rounded out by Mera and Deathstroke. The two are both incredibly strong, so traveling with them makes sense. Mera wants revenge for Aquaman, and any quarrels with Deathstroke are obviously not important when Darkseid is always lurking.
The most important takeaway of the Knightmare future is that Darkseid won. The Justice League managed to stop his takeover once, but he eventually broke through. Even without the Mother Boxes, Darkseid took over and terraformed the world. Wonder Woman and Aquaman died in the conflict, and Superman is on the side of evil. Things basically couldn't be any worse. It's unlikely that there's any saving this future. Instead, the best hope for humanity is to avoid this timeline in general. The timeline never occurring is about the only way to make sure that earth stays safe.
