I'm having the same problem detailed here:
http://steamcommunity.com/app/435150/discussions/6/1699415798766828761/Essentially, on my main save, the Shriekers in Act III are invisible and untargetable. I can see their vision ranges (sneaking and/or holding shift). And, when they see me, they kill my character(s). However, I cannot target them with abilities to be able to kill them. And it's incredibly, incredibly frustrating.
That Steam thread suggests a workaround. Happily, I have a save that I made before entering Act III, so I can actually do the workaround. Also happily, I haven't done THAT much in Act III yet, so I'm actually "only" losing about an hour of progress (which, realistically, now that I kinda know the map, I might be able to do in substantially less time). However, I'm not the only one that's encountered this bug (Steam thread) and the posters there seem to think that the problem was caused by the most recent patch. I have no way of verifying that. But, I figured I'd make a thread on the official forum, to make sure that the devs are aware of this bug... and hopefully fix it
