fog would be a command which obscures the contents of a room for several hours (exact duration ought to depend on weather). Has a timer set for about twice as long as it lasts -- can't use it again till timer wears off.

Contents of room (objects, not characters) can't be seen while a fog is in place. You can still pick stuff up using 1. Room is displayed in brief mode (plus a line "The fog is very dense here."). (Or, if that's overpowered, can't see more than one or two objects on the ground but you can still pick other things up with 3. etc.).

Can only be used outdoors, and not in rooms marked "windy" (I don't know if any rooms are currently marked "windy").

Unfogged outdoor rooms reachable from fogged room get the line "Wisps of fog drift about.", but no other special effects.

Perhaps assassins and rangers would be granted cover to hide/camo in fog even if room wouldn't normally allow this. Cloud giants could still see them, however.