The Oubliette was first discovered in the late 19th Century by The Time Traveller. This is a far-future version of Earth where all human life has died out. In this place, near the end of life on Earth, is a prison. And this is where the worst of the worst end up.
It’s in the far future because then it would require a particular talent to become a threat again. Sending enemies to the distant past always had the problem that they end up stepping on butterflies.
Notable Inmates
The Time Traveller - self-imposed exile in the prison he commissioned.