May 3rd, 2025
Three universes exist in parallel: the "demon" universe, the "gateway" universe, and the "mage" universe. Each of them started at the same time, and were linked as a result. However, they didn't all develop at the same rate, and humanity didn't evolve in quite the same way in each. Demon and mage universe humans have existed much longer than gateway humans, and also live longer. The gateway universe links the demon and mage universes, and is most like our own.
To explain, magic exists in all three universes. However, it's not as though anyone can use it; it's the result of a mutation that allows a person to absorb the energy of their world and expel it in certain ways, resulting in phenomena considered to be "magic." Mutants of the demon universe are common, and are most likely to have fire-based magic. Mage universe mutants are also common, but can employ a large number of "spells" instead of magic of a single type. Gateway mutants are incredibly rare, but also incredibly powerful. Their abilities range from simple telekinesis to the ability to control time, and in rare cases grant the user immortality.
As a result, gateway mutants are very dangerous to the stability of all 3 universes, so thousands of years ago, before humanity in the gateway universe had evolved into the form seen today, mage mutants set up a worldwide spell that locked down the magic of anyone born any spark of magic ability. The only way to override this lockdown is to die in a very specific way, and hope your abilities include a regenerative factor.
As time went on, this got pushed to the background, as demon mutants started discovering the other universes, around 2,000 years ago. They were seeking a new world, as they had long since destroyed their own. Demon mutants usually look like normal humans until they activate their magic, turning them into twisted beings commonly associated with what we consider to be demons today. They return to their normal forms after deactivation.
Mage mutants realized what was happening and decided to prevent demons from ever entering their universe, opting to fight them in the gateway universe instead. They prioritized prevention of dimensional travel, but often protected gateway humans. This started a basis of Heaven and Hell as we know it today. Demon mutants were known as demons, and mage mutants were known as angels. Occasionally, super-powerful beings came to the gateway universe, and were given titles such as God and Satan.