The Sandman Is At Your Side
Of course, while Dream hasn't historically been considered a resident of the DCU, he's kind of been established as one of its neighbors who does the occasional drive-by. Grant Morrison's mind-tripping map of the DC Multiverse that stemmed from his Multiversity series showed that Dream and his Endless siblings reside in a realm known as the Sphere of the Gods, existing between Limbo and the wall of the Speed Force. Dream has stopped by the DCU on rare occasion, as shown Gaiman and Matt Wagner's 1995 one-shot Sandman Midnight Theatre, where he crossed dream world paths with fellow Sandman Wesley Dodds, as well as a cameo in Kevin Smith and Phil Hester's 2001 "Quiver" arc in Green Arrow. Gaiman's very first Sandman arc featured a Justice League villain, in fact, and Dream's sister Death made an appearance more recently in Action Comics.
Dark Nights: Metal #1, though, is the first time that Morpheus has showed up in apparent physical form, strolling into Wayne Manor to say hi to Bruce as easily as Alfred or Dick Grayson would. In the face of the tangential connection that Dream has been shown to have with the mainstream DCU, his direct, straightforward contact with Batman is far more tangible here, carrying a more palpable sense of significance. Despite his definitive presence, the reasons behind it remain a little more dubious, beyond his ominous warning to Batman – the awakening of the ancient dark energy that now threatens existence is an obvious one, but his involvement otherwise is unknown. Whether he's simply reacting to the crisis, or perhaps is somehow involved with it, is a possibility that begs for exploration in future issues, and the teaser on the final page – "Next: Nightmares Revealed!" – would indicate that such exploration is likely.
Might Sandman Become a Player in the DC Universe?
If Morrison's multiversal map is considered DC canon, as it appears to be in this issue, then the realm of the Dreaming sits on the same plane of existence as other realms like Underworld, The Phantom Zone and Apokolips. Residents of those worlds, such as Neron, General Zod and Darkseid, have made their way planetside countless times to plague the heroes of the DCU, but Dream, until now, has remained a far quieter and more pleasant neighbor to the physical Orrery of Worlds, choosing only to perhaps invade the dreams of its denizens. Dream's reclusive nature aside, his place outside of the multiverse is no different than that of his extra-dimensional neighbors, so his ability to venture to the world of DC's heroes has already been established, and now with Metal #1, he also has his motivation.
That's not to say that Dream would necessarily be wisely used as a prolific character – he once made an appearance alongside the Justice League during Morrison's JLA run, but it's not like he's a natural fit for membership. But the character was last seen in Gaiman and J.H. Williams III's Sandman Overture, which concluded in 2015, and was only seen sporadically in the occasional comic or graphic novel before that since the final issue of the Gaiman-helmed ongoing series in 1996. The character's enduring niche popularity and relative underuse is certainly a recipe for a higher profile role in the DC Universe, at least as a periodically recurring character, and he wouldn't be the first Vertigo character to migrate to the mainstream DCU – John Constantine, another character who shares history with the Sandman, made that same leap with the onset of The New 52.
While the significance and long-term impact of Dream's uninvited drop-in at Wayne Manor will have fans buzzing for some time, there remain the already established mysteries regarding the Dark Multiverse that have only begun to be explored. Dark Nights: Metal #2 stands to address the commencement of those mysteries, along with that of the Sandman, when it goes on sale September 13.