DARK NIGHTS: METAL #2
I read the two Dark Days one-shots that set up this crossover and read Dark Nights: Metal #1 and still had no idea what the hell was happening here. No such problem in #2 as it all starts to fall into place, and it's quite simple really. Obviously Barbatos the demon bat god from the dark multiverse installed a thousands of years long plan to slowly transform Batman into the portal through which he can re-enter our dimension in order to bring a reign of darkness, and his soldiers in this war would naturally be evil Batmen from the Dark Multiverse.
Look, I can't pretend this crossover is for anyone but the most ardent fans of the entire Snyder-Capullo run (and it is clear he really has been planning this since the beginning as this issue lays out all of the ways in which this has been hinted at since Court of Owls back in 2011). It also requires some familiarity with a couple of pre-New 52 pieces of Bat-canon (Barbatos was introduced in Dark Knight, Dark City which is never mentioned as one of the all-time great Batman stories but absolutely should be and Final Crisis/The Return of Bruce Wayne are a big part as well) that had previously been seemingly discarded with the New 52 reboot. It's the kind of graduate-level canon crossover that Grant Morrison is more usually known for than Snyder, and I normally shun such things.
If you have all of the prerequisites, though, Metal is shaping up to be quite an event. I must admit that at first glance "evil Batmen from a Dark Multiverse" is a potentially schlocky premise, but the few hints we've gotten of these evil Batmen and what spawned them on their worlds are promising and the story potential of a DC Universe that has lost it Batman (spoilers, I guess, Bruce is MIA by the end of this issue) having to fight off an army led by his evil counterparts is endless. The set up is now complete, hopefully the pay off is worth it.
