I've encountered multiple bugs in the Underdark. The first one is at the cell where you first encounter Taramiel and Isivriel. Sometimes (though not all the time) the door is already open and the characters are there, but the cutscene will not play. I was able to skip them and go straight to where the characters are captured.  On a different playthrough, all of those parts went as intended, but once the part where you have to go to the Katselia weapon shop comes up, the game will not proceed further. I can go to the Underdark after that, but the whole thing is bugged. Nothing plays as intended and I get sent back to earlier cutscenes with my party still intact.