I think the problem is that my homestead got stuck; so when I met the watch is comming the imps had not yet appeared. Maybe it would be close-able if I went back there. I might reload the game just before the final fight and go back and see if it is completeable.

Any clue about the air key you get in the tomb from the earth door ? After I opened the earth door and got the air key - i did the candels and then the final two fights. I never actually found a use for the air key.
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From what people are saying the imp quest is buggy and it seems common to not be finished.
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@Finjas For lady in the lake (I got stuck on this one too but the answer is that there is a small section in hibberon (sp) forest that is blocked off by a lever. The key to the lever is nearby - if you go there you will find the lady in the lake and close the quest.