Directly from duergar's helpfile:

"Because of their many years underground, they have become adept at seeing through the shadows. However, they cannot see hidden assassins within the shadows of the wilderness."

With a bit of PK I found out that duergars seems to see hidden orcs both in caves and marshes/swamps.

As for caves, I have no problem with that (as duergars are cave creatures). But what about swamps? Should they really be able to see hidden orcs in swamps? How hidden orc in swamps differ from hidden assassin? Duergars are not swamp creatures, after all (at least I think so).

May be it would be worthwhile to make that duergars cannot see orcs while they are hidden in swamps (note: orcs can hide in swamps only during the night time).

Thanks.

n/t