(Authors note: My chapters will have a * before the option title, please don't use this so people can tell the main story branch. But feel free to add to it definitely)
Long ago in some distant age humans feared that there were many things that could wipe them out. So they build the vault. It was a super powerful system, quite advanced for the time. Today we'd call it a computer. Though it wasn't quite a computer, it could just store,receive, and later distribute information. It couldn't access many functions our computers could. A record of all mankind accomplishments so that it wouldn't be lost forever. Those who invented it stored all they knew inside of it.
There was just one catch, the computer craved knowledge and over the years evolved itself to steal intelligence whenever it could. Those who came near it had to feed it information whether they even knew it or not. They would be reduced to dumb, caveman or animal like states. While the computer had they're knowledge.
It wasn't totally cruel however, in return it gave me bigger and stronger muscles and bone structures to help them survive in this new state. Now look very primitive to go along with their primitive minds.
Luckily the vault was located deep in an underground tunnel system, to protect it from whatever would destroy humanity. It's signals couldn't go through solid rock so most people were safe. It would sometimes be 100 years or so between visitors. Lost explorers or spelunkers who stumbled across the vault.
The computers previous victims however have formed a sort of subterranean tribe in the tunnels. Protecting the vault as a god like being. Believing that there must be a powerful spirit living inside for it to know so much. Sometimes it talks with a synthetic voice and spits out what it knows, almost like rambling, just bursts of nonsense. Sometimes threading together information that has nothing to do with each other in one sentence. This is probably because of its modifications interfering with its sharing information function. But the people are too dumb to understand anyway.
Most of the children of this tribe are raised dumb and therefore have nothing to offer the computer. But sometimes a prodigy would be born with high intelligence, even without being taught anything. These children become the shaman of the tribe and when they reach maturity the computer drains their minds as well. They are still revered by the tribe as someone who's had direct contact with whatever lives in the vault.
The tribe would go outside into the nearby forest that has grown over the ruins of those who built the vault, to collect or hunt for food. This kept them from becoming some albino race from being underground forever. Every generation was still exposed to the sun. If anything they usually had tanned skin. The computer may be worshipped but it doesn't actually provide anything for them.
When someone from the outside comes in, the tribe tried to force them out. But sometimes they try to bring them to the vault as a mental sacrifice. They lose all their knowledge and end up joining the tribe too.
It is now modern times, and still no one has seen the vault and come back to our world to tell. And you are about to wander into the cave. Possibly with friends or alone. But this is your story.