Preface: This is a true dream/nightmare I had one morning a couple months ago.
I woke up, cold with sweat, and the dream was so vivid and evoked such dread in me that I still remember it in clear detail.
Many aeons ago, long before humans walked on this planet, there was a civilization of bipedal creatures similar to us. They lived in a vast underground city and were highly intelligent.
I do not know if this was on Earth or not.
Being a deeply spiritual race, they had a god that they worshiped, and they lived in peace with one another, but they were easily corrupted due to their constant search for knowledge and power.
They had everything they could want except for one thing: immortality. They begged their god for this knowledge, and it told them how to build a huge machine that would grant them this.
So they obeyed and built the machine. They started the machine and its rumbling noise could be heard throughout the entire city.
In the coming years no one would die from either wounds, illness, hunger or age. But no new children were born, and the bodies of the creatures still decayed like normal.
As time passed, they grew old and frail. One by one their bodies failed them and soon the corridors were littered with meager bodies.
They would always be of sound mind but eventually incapable of lifting even their eyelids.
They would feel their bodies crumble, their jaws falling off, their skulls hitting the floor, yet be completely helpless to cry, scream or react in any way.
There they would lay in the now completely silent rooms and hallways and witness the cracks that would form in the crumbling walls and ceilings over the course of aeons.
Eventually, dirt would cover the only parts of them that remained: Their skulls.
They would feel the constant pressure of many kilometers of soil stacked upon their heads, but their skulls would not crack,
and they would wish for death in every moment of their eternally long lives, but that release would never come.
They still lay there, deep underground, and will remain there long after the last human is dead.
I'm sure you have at some point been waiting for something, and had nothing to distract yourself with. Nothing to do but sit with your thoughts for a while. A minute feels like ten.
An hour of that is almost unbearable. Now imagine you cannot move any part of your body and then imagine experiencing that for a day. A week. A month. A year. 20 years. A human lifetime.
Thousands upon thousands of years. Maybe you would use all that time to reflect upon the world and its meaning, but you would never be able to share what you found with anyone.
You would be alone with your thoughts and knowledge. Forever for all eternity.