How to get in the undercroft (chasing echoes quest)

[cmp007]

So I found the moth priest did all that good stuff now I have to find the other scrolls. I get chasing echoes and says speak with Serana. When I speak to her she just tells me the same old stuff. So I went ahead and went back to where the undercroft is. Get to the door and you need a key. Now when im here and speak to her I get "what is this place" but after I choose that and she tells me nothing else happens. How do I get the key to open the door to the undercroft?

 

[Derek Jeder]

I think your game may be glitched. If I'm thinking of the real Undercroft theres no key needed, atleast after you get the quest.

 

[Medea]

This might like a stupid question, but are you sure it's the right door? The undercroft is on the outside of the castle.