The Jail or Prison is an area located halfway between Derphy and Yowyn and was added in 1.14fix6/1.16. It is not possible to enter the Jail normally. You will be randomly diverted when you when your karma is below -30 and you use a scroll of return. It does not seem possible to escape the jail.
A capricious controller of time has changed your destination
While there, time passes quicker and your karma will slowly go up but your attributes will decrease. You will be released from your cell some time after you reach -30 karma. You will also have to supply your own food; if you starve to death, your karma is increased by 10, as normal, but equipment you drop may not be recoverable.
You hear footsteps coming towards your cell. "Hey punk, our boss says you can leave the jail now. Do not come back, okay?" A guard unenthusiastically unlocks your cell.