Nightmare Batman explained by Zack Snyder

The Knightmare future seen in Batman v Superman :Dawn of Justice was meant to play an integral role in  J.L ( Justice league) and the rest of and snyder's five movies plan. 
and we now know a great deal about what Snyder had envisioned for this aspect of the DCEU. Since the theatrical debut of Justice League in 2017, Snyder has been more and more open about where the movie's theatrical cut parted ways with his original version.
 Snyder would also take it upon himself to put an end to the debate around his cut's existence by posting an image  to social media this past December.
Amongt themore recent developments of the Snyder Cut campaign, Zack Snyder would also provide a director's comentrary to BvS through a Vero live-stream. 
T along with an earlier director's cut event in March 2019, would see Snyder revealing new information about the making of the movie, with both offering details regarding the Knightmare seen in Batman v Superman. While presenting nuggets of information about the sequence's place within the movie itself, Snyder would also make notable reveals about how he intended to pay it off in Justice Leagueand beyond.With the Knightmare having been completely abandoned in Justice League's theatrical cut, it joins a now-mountainous accumulation of details about just how much of a 180 the film's massive reshoots made from Snyder's version, on top of an entire movie's wroth the deleted characters and other excised story points. As a result of Snyder's public comments, there's now far more context about the Knightmare's role in his planned five-movie arc.

Darkseid  in Earth 

The most readily apparent element of the Knightmare future was that Darkseid had invaded and conquered Earth. This is made clear by Omega symbol burned into the desert floor seen in Batman v Superman. However, this also ties into story points that are now known about in Justice league Snyder cut.

The theatrical version of Justice League'shistory lesson shows Steppenwolf leading the invasion, while Snyder's version of this sequence featured Darkside leading. With the Knightmare elements of Batman v Superman and what Snyder had planned to incorporate into Justice League, this would have established Darkseid had made a second, much more successful attempt at invading Earth.