Me being me, I've decided to construct a conworld (or I guess con-universe) around that game I'm developing, or at the very least make a good amount of discoverable lore. I'll keep this thread for my (probably highly disorganized) notes. Feel free to ask questions as well.
Like the rest of my conworlds, the origin story is that a pretty technologically advanced humanity fled away from their home universe into a large cluster of pocket universes known as the "Dodecaverse". The reasons for this are pretty technical, but basically the original universe was infinite and hyperbolic, which in the short-term led to expansion and in the long-term led to entire causality lines snapping off and/or being annihilated. Whole groups of people, civilizations, even whole star systems started blinking out of existence because in the universe's perspective, they never existed in the first place.
This didn't affect memory though, so humanity was able to find solutions -- those that weren't already protected from this effect in some way (like those under the influence of the Sra tree) decided to flee into universes with parabolic or flat geometry -- there's a particularly large complex of these that spreads out in 12 dimensions known as the Dodecaverse, many of which had stable (albeit weird) physics systems that could support life.
The Shatterloop was one of those. Known as the time as The Loop, it was a finite plane that reached back around and intersected itself after ~9 quadrillion km (about 7,000 light-dog-years, iirc) in two dimensions, and collapsed into a singularity in the other two directions after not far at all (like 500 miles in either direction). While these singularity-type dimensions were dangerous, the physics were set up very very well -- the second singularity had a gravitational pull exactly 9.79m/s stronger than the first one, which means gravity was about perfect provided you had enough countermass to keep your landmass afloat. Like many other habitable universes, there was a band of self-illumination -- no need for a sun that could explode at any minute, instead you just get a nice sky that provides light for you. As an added bonus, earth technology seemed to work most of the time, so humanity was able to transport things slowly and maintain their level of civilization.
The final nail in the coffin for not going though was the ease with which matter could be converted into energy and vice-versa -- that meant that building things just required energy rather than building materials, and if you needed energy you could just tear down useless structures to provide it. You could also expand your planet given enough time and energy -- which the self-illuminating light band would provide.
In the end, they decided to move a Conical Ark -- a cone-shaped landmass with the pointy end sticking down. A lot of civilizations sent these out because they were quite easy to create from planets (just detonate the core, and there you go). So they gathered in one big continent on their planet, detonated the core, and used the ensuing energy to transport the whole thing into The Loop (with a lot of shielding of course).
For some reason, though, when this happened, space shrunk around the Ark so instead of having a big cone surrounded by void, you instead just had a continent-sized landmass that looped back on itself. The Shrinking, as it was called, also knocked out a bunch of technology, particularly the ability to move back into the old universe.
What technology still worked was used to construct six enormous machines known as Pivots to probe spacetime from slightly outside of it and figure out what the hell was going on. What they learned was that the universe was still a thousand light-years wide but anywhere there was matter, spacetime would curve slightly, with more matter meaning more visible curving. Meaning that the singularities themselves were probably the reason the universe was so small -- though they couldn't find a space outside of it. Horrifyingly though, this curving wasn't stable -- meaning that any time the land could rip apart as space decided to randomly expand.
Working feverishly, they spread the Pivots out and used them to turn energy into mass or vice-versa to correct these ebbs and flows. In staving off these natural corrections though they caused something much much worse which they didn't realize until it was already happening.
Their little shell of a world expanded across the length of the far-more-stable 1,000 light year universe, tearing their cities apart, generating so much energy that new land formed in the cracks, and otherwise just heavily scattering things.
If that's not bad enough, all causality itself became Uncertain -- with the exception of the Pivots, the one expanded and chaotic universe split into 2^54 variations of itself, some with different physical laws and even inherent Uncertainty themselves. With some exceptions, technology failed or was just way too scattered across this new enormous multiverse to be useful anymore. People also split into 2^54 copies, becoming hopelessly confused as their memories split off into whichever copies could actually retain them. People forgot who they were, where they were, how to do things and basically had to relearn all of the above. Crops scattered and changed, domesticated animals mutated into feral or hideous monsters, the very physics of almost all materials changed drastically on a dimension-specific basis.
Over time, people were able to pick up the pieces, though the damage had been done. What technology could be found was used to try and rebuild civilization again -- ways to move to far physically-distant places, ways to move to adjacent dimensions, ways to probe at materials and figure out how to use them again. At the time the game begins, people are this strange mix of medieval agrarian and connected by portals (some interdimensional) to a vast network of other agrarian societies. For over a hundred years since the Expansion, people have had no idea whether they're trading with other towns or other versions of themselves.