Nulfaga refers to the Mantella Crux as the 'prison' of the Mantella. She herself embraced her insanity. It actually seems to be Sheogorath's realm, or a small part of it. The first signs are small such as rooms with the furniture on the ceiling ala Alice in Wonderland. Other than this its for the most part just a random series of unrelated seeming styles. Space with floating rocks, doors that are connected to nothing, and an aztec pyramid. A 'Benefactor' as he calls himself dressed as a dark brotherhood assassins stood in a large room with just a small bed and two opposit gender dead bodies, rooms made entirely out of fire and walkways, and traps that randomly place you at a great height so you can 'enjoy the view'.
Then, the sole person other than this otherwise none descript 'benefactor' that you meet is Sheogorath himself. He says the following...
"Ah, at last! I was beginning to think you wouldn't get this far. A dreary place, don't you agree? And guarded by a pompous upstart-- he's no more a god than I am. Well, carry on. We will meet again soon, unless you are not as foolhardly as you appear."
He speaks of a guard; but all you find next is a skull statue that asks you a question to pass. Not what i'd call a guard; then we run into him again a little while later, one final time. I'm tempted to think he was speaking of himself and that he was just making a funny with his 'no more a god than I am'. He is in a sitting room and you have to answer him to move on.
"A familiar voice laughs mockingly, "So, ---, come to pay your respects at last?
"Greetings, ---. You dare much by intruding here. Nevertheless, I will allow you to proceed, provided you prove as perceptive as rumor reports. You may be aware that I am a master of the blade. But what is my other favored weapon? Answer correctly, then arm yourself."
The answer was a crossbow, just incase anyone was curious about this quirk.
The dungeon ends a short time later.
So, aside from this nameless and otherwise unknown being (as far as I myself know anyhow) Sheogorath, who otherwise ahs no reason to even be there, is the only other Named Entity than you in the place, which at times shares a lot of his 'madness'.
Just curious on peoples thoughts. Was the Mantella Crux a pocket of the Shivering Isles?