So, I got to thinking.

According to Skyrim, the Skeleton Key needs to be in the Twilight Sepulcher, and there are Nightingales to ensure that it stays that way.

According to Lore, the term "Nightingale" has existed dating back to the era of Jagar Tharn, specifically during the adventures of Barenziah.

Now, in Daggerfall all the way to Oblivion (IIRC), it's possible to obtain the Skeleton Key.

Does this mean that Nocturnal removed the key from Ebonmere and gave it to the Eternal Champion/Nerevarine/Champion of Cyrodiil, thereby closing the Ebonmere, rendering Skyrim's Thieves Guild to fend for itself?

Ugh, horrid Nightingale lore.

Who's to say there is only one key? Nocturnal is the Ur-Daedra, she seems capable of having a "master key" and obtainable copies for worthy servants. Even if she isn't capable of making a good deal.

Where's this coming from that Nocturnal is the Ur-Daedra? I'm pretty sure that title doesn't belong to her.

http://www.gamesas.com/topic/1327177-tatterdemalion-the-lunar-province-of-secunda/ mentions Nocturnal acting as the Ur-Dra (and thus getting all the Daedric Princes to agree to not meddle with Reman's moon colonization efforts).

It's probably mentioned elsewhere as well, but as I just reread Tatterdemalion it's the only one I can think of off the top of my head.

I see. I would have thought that Namira would have that title. I mean, her sphere is the "Ancient Darkness", and her realm is "The Scuttling Void". Thematically, I think it makes better sense.

http://www.imperial-library.info/content/nocturnal

The first paragraph originally comes from a Michael Kirkbride forum post that I'm too lazy to try to track down right now.

EDIT: On a somewhat tangential note, could Sithis be, in fact, a genderbent Nocturnal? She would fit the bill, being a personification/aspect of the Void (assuming that the whole Ur-dra thing isn't complete bull[SEHT], of course).