Had a dream about a civilization on an island with part of a mountain in space where I discovered a cult

I began my venture into this world nonexistent. I watched a small civilization go through the effort of continuing to exist near the base of absurdly tall mountains. They threw magic around to grow trees and crops for their houses and food. Their capitol city was quite large, for a society with a magic-laden medieval-esque technology level. People went about their lives with no knowledge that I was about to appear at the base of the mountain range.

Which I promptly did. I talked to a few people in town and wondered what was on the other side of the mountains. No one knew, so I set off to find out. It wasn't long before I'd made it through the lowest point between two peaks I could find within a few hours, and I saw nothing.

Not an empty, barren landscape. Not plains.

Nothing.

The world beyond the mountains didn't exist, and the mountains themselves had scars indicating having been sheered off, sudden changes in slope to fully vertical, or even receding at some points. My view zoomed out from myself and I saw the truth: The land of this world was just this island, plucked from wherever it may have been and thrown into space. How gravity still worked, I didn't know, but did wonder.

Regardless, the ground under the island came to a point a few miles down, like someone had taken the world's largest shovel and dug full circle around this land before plucking it out and chucking it into space. I went back and explained this to the few people I'd talked with, and rapidly got taken by the police. Apparently, this was a secret that very few people were privy to, due to the panic it had caused generations ago when it was common knowledge.

They had some survey teams working on tracking the world's border because it was shrinking. Every few minutes, a handful of dirt finally decides it's time to let go and falls off the world. Sometimes, it's a whole avalanche. I decided to join that effort as a solo team and went back out into the border to watch as dirt occasionally fell down, while also seeing if there's anywhere interesting to explore.

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I watched as part of a mountain opened up and some lava spilled out into the void below. That was apparently something that'd never been seen happen before. A little bit of rapidly cooling lava remained in a small pocket at the bottom of the world, though it led nowhere. How it got hot enough to form magma, I had no idea, but this place was interesting, so I stuck around and explored the cliff faces a bit.

Long enough to find a cave.

I turned to report my findings when the ground opened beneath me and I fell into the caves under the island, right onto a metal platform held aloft by chains. Around me, hundreds of bodies were pinned to the walls, all with tortured faces and bodies, and all dead. A massive machine in the shape of a humanoid torso, neck, and head stood watch over the platform and told me I would die here, like the rest of them.

I ignored it and jumped down the pit around the platform. The machine was apparently surprised by my non-reaction, as it was too slow to catch me before I fell out of reach. I landed in a cave system lit by electricity with the occasional door and window in the wall, leading to front rooms of futuristic houses.

This was apparently the housing section of some sort of underground society. Some people tried to kill me, but I was an outsider to this world. Unlike many other worlds I occasionally travel to, I did not temporarily inhabit a body belonging to the world that just so happened to be identical to the one back home. I was well and truly an outsider, as their attacks passed right through me.

After a little convincing, I got told the true history of the world.

Some guy was traveling the stars when his ship crashed into itself, a problem with their literally magical FTL that had thought to be fixed long ago. Stranded in space, he used the gravity generator of his broken ship as a base to expand out an island with earth magic. Some air magic to create an atmosphere and life magic to create plants and he had a livable habitat to wait for help that never came.

Eventually, humans evolved on his island and found his wreckage and learned how to operate the computers. A literal cult started around this alien and his technology, which was used to create a small, hidden society where they'd slowly figure out how to create everything the alien had had and more, then ride out into the galaxy and raze the world for never saving their effective creator.

I'm not sure how this led them to creating the murderbot guarding this place with the hundreds of dead bodies as a deterrent, but I never found out before I woke up.