Jyggalag never had his own realm till he was 'transformed' (I use that would in a general sense), into Sheogorath. He had a sphere of influence, but not a realm. He however attacked/took over realms, apparently in a similar fashion to what we see happening to the Shivering Isles (the land becomming 'ordered').

Neither is a transformation of a Daedra into an Aedra.

According to http://www.imperial-library.info/mwbooks/varietiesoffaith.shtml Sheogorath is thought to be of Aldmeri origin. He is "born" when Lorkhan's divine spark is removed, and is also refered to as the "Sithis-Shaped Hole" of the world.

So, how might Sheogorath being the product of the removal of Lorkhan's divine spark, and also the Sithis-Shaped Hole, correspond to his identification with Jyggalag? (I have some loose ideas about answering this, but I don't have a complete "answer".)

Also, the connection between Sheogorath and Lorkhan suggests that recent occurences in the Shivering Isles may not be unrelated to the Oblivion crisis.

Maybe Jyggy can just be room mates with Peryite? Those two would get along famously.

This answer seems to imply that "the daedra are vastly more complex and ineffable than your laughably simplistic myths and legends inform you. Do not try to find patterns understandable to the mortal mind." I read this as a rather direct statement from the SI on authors on what led them to write something incompatible with prior knowledge on Sheogorath. What, with respect, the authors don't get, is that jettisoning a flawed mortal understanding of the history of the gods and telling us "it's too awesome and mysterious to comprehend" instead of providing another subjective, interesting source, is not good storytelling, not good characterization, and not good lore. It would be like MK coming into the 'World Eating 101' thread, telling us the Monomyth and Anuad were wrong, posting nonsense in Esperanto and then leaving.

There is the mysterious, and there is the cop-out mystery. I admit I haven't played or paid much attention to SI, but I don't think I missed anything huge.

Well there is the last line that turns everything on it's head, but it's a non answer in anyway you look at it. Either it is a cheap excuse that isn't worth reading, or as the last line suggest everything does revolve around Mundus and there is no explanation for Jyg.

Something that really confuses me about Jyggalag/Sheogorath is the fact that at the end of SI, the CoC supposedly BECOMES Sheogorath, or at least an avatar or some other form of his being(as I don't really feel like a real daedric prince much with the rewards at the end of SI). What I don't understand, is at the end, you 'release' Jyggalag from his curse of the Greymarch, and he seemingly disappears while you become the next Sheogorath.

This may just be my own interpretation, but when you 'release' him from the curse the other daedric princes put on him, wouldn't that destroy the SI and the part of him that IS Sheogorath? From what I've gathered, Sheogorath and Jyggalag are one in the same. Also, its my understanding daedric princes can't 'die' in the mortal sense, so how in the hell can Sheogorath and Jyggalag exist at the same time? It just seems like they play upon the idea that they are the same person in lore and in the SI expansion, yet at the end, they seem to become two separate entities, since Jyggalag can't just simply 'die' while you're supposedly running around as the next Sheogorath.

P.S. I'm with pieterjan on the Sheogorath playing a prank on the CoC theory lol. It fits his sphere too perfectly

If it's not a prank then the elaborate ecosystem of the SI combined with "madness as source of power" might come into play, but that has only been hinted ever so slightly...

The Shivering Isles requires Lord Sheogorath to be the Shivering Isles, not Jyggalag. Since there is a new Lord Sheogorath, the Plane of Madness can continue to exist. You're right. Up to a point they WERE two sides of the same coin, but something unprecedented has happened and for the very first time they both exist embodied in separate entities. The madness part of him was "exorcized" into the CoC, leaving only Jyggalag.

In my opinion this may have even weakened Jyg and he may not actually be a Daedric Prince any more. But that's just a hypothesis. He doesn't appear to have a realm of his own, but this might be remedied. It seems impossible for me to comprehend that there could be seventeen Daedric Princes, as that would throw off the whole spaces between the spokes of the wheel thing. (Eight Divines, Sixteen Princes...but Talos already messed up that whole harmony, it seems.)