Anyone know where this is at? I'm currently dead in the water with the moth priest aggressive bug and Serana won't come out of Dawnguard castle. I'm on Xbox 360 and starting over isn't an option.

Well you should have made sure you had an early save game to go back to. I do not remember ever needing an Undercroft key, so I assume you are trying to do the quest out of order or it has stuffed up some other way. You are not getting in there without Serana for a start, so you better go pry her out of that castle.