Llahlahk'je submitted this

Once you develop torture chambers, it
may be worth the risk to forego letting
prisoners rot away in prison, and instead
attempt to convert them to your cause
and beat information out of them in the
chamber. The negative is there is a chance
they will die and return as a measly ghost,
but imps turn very easily, and others turn
as well, and it would certainly be nice
to gain your opponents level 7 Bile demon,
now wouldn't it? :)

Another secret level? ...let me hear about it!