Rise of the Dark One
The Prison is located deep in Tiefling controlled territory. It is surrounded by a magical sandstorm that never ends. Only those attuned to it can navigate it safely. The convicts inside are traitors, murderers, and those who have angered the Tieflings. Escape from it is near impossible. Tieflings have utilized state of the art technology and magic to enforce its security. The cells are all individualized cages hanging from a complex system of levels. Not much is known about it, even to its own residents.
For purposes of plot, you must decide why you are locked up in Darkstone Prison. It has to include a strong reason why the Tieflings would want you locked up. Clear any choices with the DM before making anything permanent.