The Dark Multiverse is a shadow under the primary Multiverse, one formed of every fear and bad decision ever made or dreamt. Worlds in the Dark Multiverse are fragile and constantly deteriorating, inevitably doomed to destruction by their own malformed construction. Contrary to the worlds of the normal Multiverse, those from the Dark Multiverse have their name preceded by a minus sign, such as Earth -52 compared to Earth 0, for instance. Each of those inverse worlds shown so far bears some degree of thematic resemblance to its positive counterpart.
It wasn't always the case however. At the dawn of creation, the Dark Multiverse was known as the World Forge, the realm of a cosmic being known as The Forger, the equal to the Monitor and the Anti-Monitor. It was there that worlds were created out of the pure metal of creation, the stable ones joining the Orrery of Worlds while the unstable ones were destroyed, their components used again to create more worlds. This destruction was the task of Barbatos, then known as the Great Dragon. After eons serving his master, he rebelled against The Forger, killed him and turned the World Forge into today's Dark Multiverse.
Because of its nature as the cradle of yet unborn worlds, number of worlds in the Dark Multiverse is far greater than in the normal Multiverse, as seen when Barbatos chose to show to Batman the "galaxies" of worlds born from his fears. Other heroes also have had an impact on the Dark Multiverse, with many twisted versions of heroes such as The Flash, Aquaman, and Wonder Woman preparing to invade the Multiverse, even if they could have been created by Batman's fears all the same.
First found in the prime Multiverse on Prime Earth by the Blackhawk Squadron and the Challengers of the Unknown, the Dark Multiverse's agents, the Court of Owls, engineered the "mantling" of Batman using the five heavy metals of the Dark Multiverse, Nth Metal, Promethium, Dionesium, Electrum, and Batmanium, sending him to that corrupted multiverse while bringing forth their master, Barbatos, and his Dark Knights formed of Bruce Wayne's nightmares.
With the help of the Multiversal Tuning Fork, Steel, the Flash, and the Phantom Zone Projector, Superman infiltrated the Dark Multiverse in search of Batman, only for his entrance to be part of a trap by the denizens, who hooked him up to the same machine that Batman was in order to use him as a battery to drive the Multiverse into the dark, letting loose Superman's own nightmarish versions in the process.
Known Earths in the Dark Multiverse:
Earth -52
Earth -51
Earth -50
Earth -49
Earth -48
Earth -47
Earth -46
Earth -45
Earth -44
Earth -43
Earth -42
Earth -41
Earth -40
Earth -39
Earth -38
Earth -37
Earth -36
Earth -35
Earth -34
Earth -33
Earth -32
Earth -31
Earth -30
Earth -29
Earth -28
Earth -27
Earth -26
Earth -25
Earth -24
Earth -23
Earth -22
Earth -21
Earth -20
Earth -19
Earth -18
Earth -17
Earth -16
Earth -15
Earth -14
Earth -13
Earth -12
Earth -11
Earth -10
Earth -9
Earth -8
Earth -7
Earth -6
Earth -5
Earth -4
Earth -3
Earth -2
Earth -1
