Invisibility, before it bugged out for me and only enemies could practically use it anymore, usually cost them their turn to investigate. Though, maybe not. It can be difficult to spot when going invisible near a ton of enemies, but I think I remember it being pretty good in wasting Duergar's time. Don't quote me on that, that was half a year ago.
A good counter to NPC invisibility appears to be the wet affect, since that gives them the same bug it does me. Say goodbye to your invisibility for 3 turns, hah!
