In one of the many [Batman v Superman: Dawn of Justice](tag:711870) trailers, Batman is seen staring down a wasteland marked by an Omega sign. Darkseid is a common user of the Omega seal as a way to mark his destruction. When this image was released a few months ago, everyone spoke about it but no one seemed to speculate about how Batman would be aware of an alien being enough to dream about it. Here's a few of my EXTREMELY unlikely theories about this.
My first idea was of course, another alien. Specifically, J'onn J'onzz, or also known as Martian Manhunter. Me being a huge fan of Manhunter, praying for him to find his way into the Justice League films, I've produced the wild crackpot theory that Manhunter has been providing Batman with information of alien forces for year. In an attempt to prove my theory, I'd like to point out two missing key members from the Justice League concept art. Martian Manhunter and Green Lantern. Both of these characters consistently deal with alien forces.
Whether or not these characters are being saved to be revealed in Justice League or not, these characters would be pretty ideal for a battle against Darkseid. These two are also the only ones who could have information of Darkseid to alert Batman of him. Except one other person of course.
Maybe Cyborg has come back from the future that is Batman's "knightmare" and warned Bats of Darkseid's reign. Cyborg and/or Flash using time travel would be my last two guesses for who'd have the resources/abilities to warn Batman of Darkseid.
Let me know of your crackhead theories that your yarn trails have lead you to!