The scenes from the trailers that consistently have people scratching their heads are the post-apocalyptic sequences. It features everything from a ravaged landscape, soldiers loyal to Superman, what looks to be parademons, and Batman in some sort of desert costume. The whole thing feels very out of place from the rest of the film and has lead to suggestions that its not really happening. This thought has been corroborated by multiple sources. The first was costume designer Michael Wilkinson, who confirmed it to be a vision of sorts. This fell in line with the leaked name for the action figure of said desert Batman, which was called "Knightmare Batman." So we know that what's occurring isn't real, but what's causing Batman to see it? Perhaps a fear toxin of some kind, Lex Luthor is a noted inventor and has a penchant for experimental weaponry. Maybe Bruce Wayne gets knocked out during the Battle Of Metropolis and has a prophetic vision that leads him to take up his fight against Superman. Or, perhaps most likely, it's as simple as the title suggests: a nightmare. In addition to how he sees these things, it's also important to remember that parademons are the footsoldiers of Darkseid, and with him thought to be the villain of Justice League, the vision could be foreshadowing more than just the events of this film.