Cad3x: Spoilers ahead.

In Dungeons of the Forsaken, I heard the countdown and the horn. I was located above the acid pool, at the lever. However, even after 10 minutes, nobody came. I loaded a savegame from the start of the level, tried it again. Same result.
Is there any trigger I have to know?

SCPM: I had the same issue, I ended up loading the level, using the 'Restart level' option from the load screen, then playing normally until the attacks began properly. It's possible that loading a save file before the first wave caused some kind of issue.