This is a version of Earth where humanity was put out of its misery. It exists in a permanent technological dark age— the planet is colder, the oceans are lower, and the natural world has grown unimpeded for several centuries. It is also a reality with an unnaturally high concentration of magic. Monsters and mythical creatures are commonplace. Rare plants grow strange and twisted fruits. Enchanted farmland creeps across the valleys and the riverlands. It is a bizarre, untamed world. Most humans who travel there do not make it back.
Something went wrong, and their universe snapped. The outer shell of reality was cracked open, and millions of connection points were formed between our universe and theirs. In the process, our world was exposed to magic for the first time in thousands of years. In their reality, the event is called an "existential hemorrhage." In ours, we call it the Eclipse.
With the assistance of magic, it's possible to travel along these connection points to reach Anotherworld— and from there, to anywhere else in the connected universes.
There are humans on Homeworld who are desperate to take Anotherworld's resources for their own— namely, the United States government. On the other side, a coalition of magical entities has made it their duty to strike back against their would-be conquerers, and take over our Earth in the process. There have been three Anotherworld Wars, all carried out in silence, with very little push from either side. They may be testing the waters, or we may have realized that it was not a good idea in the first place.
At the moment, the conflict has reached a stalemate.