When Perpetua created the Multiverse, she shaped it in a three tripartite form- positive matter, firm and constant, antimatter, corrosive and all destroying, dark matter, rich with potential. The realm of dark matter is the World Forge.
The World Forge is the realm of dark matter, rich with potential, beneath creation, from which all the worlds in the Multiverse and the Dark Multiverse have been created. It was commanded by Alpheus, the World Forger, who was tasked to create new worlds by his mother, Perpetua. He then kept working at the Forge after Perpetua was imprisoned.
When Perpetua created the Multiverse, she shaped it in a three tripartite form- positive matter, firm and constant, antimatter, corrosive and all destroying, dark matter, rich with potential. The realm of dark matter is the World Forge. After the creation of her Multiverse, the World Forge was given to her son Alpheus, so he could create new worlds out of it using Element X, combined with the hopes and fears of all living beings. Some of these new worlds were born stable, and so they ascended to the Orrery to become part of the Multiverse. Others were unstable and twisted, and so Alpheus created a servant, the dragon Barbatos, tasking him with destroying all those worlds.
But the dragon knew only destruction and pain, so he decided to betray his master, killing him. He then stopped destroying the unstable worlds and the Forge itself begun its darkening: Barbatos looked upon a new Dark Multiverse, made of nightmares and catastrophes.
Dark Nights: Metal
With Batman and Superman imprisoned by Barbatos inside the Dark Multiverse, Dream intervened, answering Bruce's calling for help, and transported them to the Dreaming, inside Lucien's Library. Once there, he explained them what the World Forge was, also revealing the Forge was darkening, with its fires soon destined to be extinguished. The Element X inside the Forge and its own re-activation could be the only key to defeat Barbatos, or so Daniel thought. Powered by belief, Clark and Bruce decided to step inside the passage Daniel opened for them, getting inside the Forge.
Once there, they both witnessed the fact that the Forge was practically shut down. Also, a new dragon, created by Barbatos, stood as its guardian: it was a twisted version of Carter Hall.  Later, Wonder Woman also understood how to access the Forge, hearing Superman and Batman's calling. Opening a portal to the Forge thanks to Hawkman's Mace, Diana was also able to make Kendra and Carter talk, with Hawkgirl making Carter realize who he truly was, so that he could step out of his role as Barbatos' dragon. Then, Batman, Superman and Wonder Woman were able to find the last Element X present in the Forge, and thanks to it and their belief, they were able to reignite the World Forge.
Doom Rising and Death Metal
After Perpetua was freed up and fully in control of her powers again, she took back the Multiverse under the power of Doom, and tasked the Anti-Monitor, now reformed under her power, to become the new Forger, to create worlds that would have Doom at their core.