When Batman shot Darkseid with a radion bullet at the end of Final Crisis, the dark god retaliated by striking Bruce down with his Omega Beams, seemingly killing him. Earth's heroes mourned the loss of one of their finest, but in reality, Darkseid had not killed Batman. He had used his powers to warp time around Bruce, infusing him with Omega energy and sending him back to the Stone Age without any memory of who he really was.
The infusion of Omega energy would cause Bruce to uncontrollably jump forward in time until he reached the present. Each jump would increase the concentration of Omega energy in Bruce's body, until his final arrival in the present, at which point the energy would blow a hole in time and decimate the universe. Once this happened, the Hyper-Adapter would take control of Batman's body, carving out a space in his mind for Darkseid to return, take possession of it and rule over reality once and for all.
In 1765, a young Thomas Wayne, along with numerous other influential colonial Americans, participate in a demonic mass where they attempt to summon the demon Barbatos, in an attempt to gain eternal life. When the summoning seemingly works, all of the men but Thomas flee in fear, locking Thomas underground with the demon. While Thomas believes he had summoned Barbatos, what actually appeared to him was Darkseid's weapon, the Hyper Adapter, which adapted to Hurt, with the intent of using Thomas as a weapon against his future descendant.
In the earliest days of existence, before the Multiverse became reality, a spark of potential created matter and anti-matter. After that, two siblings tasked with monitoring creation were born. Along with them came a third being known as The Forger. Unlike the others, it was set to watch over creations that would exist in the future. The Forger made its home in the Forge of Worlds, a hidden place inside what would later be named the Dark Multiverse. For eons, it created universes based on the hopes and fears of every living thing. While the stable universes became part of the Multiverse, The Forger would dispatch its dragon to destroy the unstable worlds so their energy could return to the Forge and be used later. The name of that dragon was Barbatos. Eventually, Barbatos' lust for destruction led him to kill his master and corrupt the World Forge. Because he was no longer doing his job, the unstable worlds persisted beyond their time and became part of the Dark Multiverse.
Eventually Barbatos set his sights on destroying the regular Multiverse. He was able to come to Earth at one point, and when he did, the Judas Tribe worshiped him as a god. However, he was repelled from the positive matter Multiverse one of Hawkman's ealier reincarnations. When Batman was sent back in time by Darkseid with the Hyper-Adapter, he was noticed by Barbatos, who saw the similarities in the emblem that they used. Barbatos decided that he would turn Batman into a portal that would allow him access to the regular matter Multiverse in order to control it, with Hath-Set as his first conduit. Some time later, he managed to capture Hawkman in one of his many incarnations, and named him his dragon, mutating him into becoming defender of the Forge of Worlds.
Out of the Dark
Many centuries in the future, Barbatos' followers started to secretly prepare Batman to become the doorway for Barbatos to enter Prime Earth from the Dark Multiverse. His tribe of followers, now known as the Court of Owls, planned to infect his body with five heavy metals from the Dark Multiverse. Learning of this, Batman tried to go back in time and stop Barbatos once and for all. However, Batman fell into a trap and was injected with Batmanium, the last metal needed. Batman was transformed into the doorway which allowed Barbatos and his core agents to come from the Dark Multiverse. At this point Barbatos had his subjects kill the Court of Owls before personally defeating Superman and Wonder Woman.
In the Dark Multiverse, Barbatos put Batman under the illusion of being an elderly man reading a story about his adventures from his time as Batman to his granddaughter, Janet. However, Bruce noticed that his memories and the story didn't match, and this allowed him to break free from the illusion. Barbatos then confronted Bruce and told him that the only reason Batman had become a great hero is because he willed it, that he had shaped Bruce to became Batman and that he is nothing without him. Barbatos informed Bruce that the Dark Multiverse had created thousands of worlds built by his fears, and he picked the darkest among them to be sent into the Prime Earth, and that they were winning. Faced with these revelations, Bruce surrendered to Barbatos.
Spreading the Dark
While his Knights spread chaos across the world, Barbatos mainly hung back across Challenger Mountain in Gotham. The Batman Who Laughs would periodically upadte him on his allies' progress.