SPOILER WARNING: The following article contains major spoilers for Dark Nights: Metal #4 by Scott Snyder and Greg Capullo, on sale now.
The concept of a multiverse has always historically been a central element of DC Comics continuity, but eventually had become too much of a quagmire for its writers to handle. In response, the publisher eventually initiated its first-ever "event" story – Crisis on Infinite Earths – to try and plausibly get rid of it, but once done, was missed by many. With some more careful planning, the idea of a multiverse was ultimately reinstated, and in fact has been enhanced with even more complexity.
Dark Nights: Metal has played a large part in that enhancement, with Scott Snyder and Greg Capullo introducing the notion of a Dark Multiverse, residing on the flipside of the already known multiverse. At first pass, the addition of a whole new realm based on the fears and nightmares stemming from the known worlds is seemingly poised to further complicate continuity. However, Dark Nights: Metal #4 goes a long way towards doing just the opposite, by not only explaining the origin of the Dark Multiverse, but just how the known multiverse fits into the bigger picture of DC continuity.
The cliffhanger ending to the first issue of Metal integrated perhaps the most surprising DC character of all into the story: Morpheus, aka Dream, aka The Sandman. Dream's appearance signaled that there was more than just another multiverse to deal with – there was also the far larger fabric of reality to consider, at least within the confines of DC continuity. In Dark Nights: Metal #4, Dream not only returns, but also takes on a more active role within the context of the story. Having been trapped in the Dark Multiverse and unwittingly luring Superman there, Batman calls upon Dream for aid, and The Sandman complies. The two shortly find themselves transported to The Sandman's realm, specifically in the library of Dream's servant Lucien.
Sandman readers will recall that the library within the realm known as The Dreaming contains every book that was ever written – even if only in the writer's imagination. Among those stories are those that were never meant to be, and should never have come into existence – stories that Dream calls "horrors from the human heart." Dream tells Batman and Superman that these imaginary, horrific stories are the foundation that the Dark Multiverse is built upon, and it's Barbatos' machinations that are giving these tomes a life they never should have had.
As these horrors come to life, The Dreaming's library begins to burn, making Dream's motivation for taking action clear: he needs the world's heroes to prevail, so that his realm, and all of reality, can be saved. Dream's removal of the World's Finest duo from the Dark Multiverse has momentarily prevented Earth from being drawn any closer to the dark realm, but Barbatos remains positioned for victory. To help the heroes defeat Barbatos, Sandman provides the two with an important piece of information: the origin of the Multiverse.