The Darkness Returns quest cannot be completed. When you get to Nocturnal, it says to listen to her - but it doesn't matter if you click 'talk to Nocturnal' or not, she just talks and then sinks back into the ground. The quest pointer sinks underground with her and the quest log still says to 'Listen to Nocturnal.' According to Wikia, at this point I'm supposed to talk to Karliah, but she doesn't accompany me on this quest (her dialogue specifically states this - and the conversation with Gallus indicates she's not supposed to be with me either). Wikia also has this same bug reported for 360 users, so maybe it's just on the console versions.

Fortunately I have a save file before I got stuck in this room, but this character is my thief and I was hoping to complete all the major Thieve's Guild quests. I guess I'll just hang on to the Skeleton Key until a patch fixes it. I'm crossing my fingers that 1.2 will, but I can't find any information on what's actually in that patch (anyone know?).

I completed this quest fine on the PS3.

So doesn't happen all the time.