In order to make a room dark, just set the lock on the room. If 
anything or anyone in the room passes the lock, then the room will 
act normally. If everyone and everything in the room fail the lock, 
then whenever the room's description should be shown, the room's 
@fail (and @ofail, if set) will be displayed instead. Also, most 
objects and exits won't appear when you LOOK, and won't be accessible
(objects won't be able to be picked up, and exits won't be able to be 
traveled through).
If you want an exit or object to be visible in a darkened room, and 
for it to be usable, then set it VISIBLE. If you want it to be usable,
but to still not appear in the room, then you can combine the VISIBLE 
flag with the DARK flag.