The cell was an air bubble 20 leagues below the oceans surface. A simple rock structure with a small grated hole in the bottom. There was no other entrance or exit. The walls were mold covered stone with no cracks or divots. The few bits of floor that were not under water were also covered in mold.
The cell's single occupant slept in one corner that was ever so slightly higher than the rest of the room. He drank from a small leak located in that same corner. He ate curious fish that came up poking puckering mouths at the waters surface. He rounded off his diet of fish with scrapings off the wall.
Every morning was the same routine. Wake up, do pushups, wallstands, curlups, jumping jacks, every varation of exercise imaginable. After roughly three hours, sit in meditation for a few hours, try and catch a fish. Nap until the daily dose of fresh air. Repeat. Stretch deply before bed. Stare at the ceiling until sleep comes.
Every five years frustrated granite claws slash at the walls and screams echo in the chamber in a brief display of emotion and grief before the stoicism returns.
But in the empty dead city beneath his cell there are no souls to hear his cries, to one to respond to his anger.
The city died a long time ago. Now only automated golems tend the prisoner, driving rocky geofish towards his cell, cultivating the oxygen releasing plants to keep him breathing.
The illex* who once lived here are all mindless plant zombies, infected by a single poisonous flower brought into this city by an unknowning offworld visitor; himself immune to poison he thought of the effect it would have on other beings. He only thought of presenting a rare jewel to his love.
She died 3 hours after receiving it. He was immediately accosted and arrested, locked away before the unthinkable happened.
The poison spread through her dead body and resurrected a plant clone whose tenticles now boasted long poisonous thorns dripping with with the very same poison that killed her.
With the single minded purpose of a creeping vine the plant sought out other victims. Within days the city was completely in chaos. Within the week all trade was stopped. The city interdicted. The tunnels leading to it closed. Everyone left succumbed. In a month, only plantlife, golems and geofish lurked where illex had made their homes.
That was centuries ago.
None of their screams had left the watery world they lived in to echo into his cell.
And so each half decade he raged in his mad calm not understanding what happened and who was to blame.
*An illex is a sentient similar in most respects to a squid.
I wrote this as an RP story for a game I play. It is set on an ocean world deep beneath its oceans.