After the player becomes Lord Sheogorath and the Champion of Cyrodiil, it occurs to me that Mehrunes Dagon might be sufficiently annoyed to stir up trouble in SI. He's currently locked out of Tamriel, but I'm not aware of anything restricting him from sending his minions to be unpleasant in the Isles. There's the question of how he becomes aware of CoC=Sheo, but maybe Jyggalag mentioned it in the bar the other night.

The game leaves Mythic Dawn acolytes and guards respawning in the Lake Arrius Caverns, so they don't appear to be too concerned that Mankur Camoran isn't acting as Dagon's representative any more, and could potentially be continuing to help Dagon's cause (which I assume is basically destruction for its own sake, rather than Camoran's overlay of conquest). They wouldn't be travelling to SI themselves, but would continue to act as spies and such in Tamriel. Dagon has no following in SI that I can think of, but does he need any to be able to open gates there, and send in his Daedra? Could he twist the Zealots or Heretics to his use?

Heretics Dislike Sheogorath and prefer Arden Sul. Zealots love Sheogorath a little too much. So Dagon might be able toconvince the Heretics. But there are a few problems with an attempted invaion:

1: The Isles ARE Sheogorath, and Sheogorath IS the Isles. So The champion of madness (now Sheogorth) can chose which laws of physics and etc. he/she wants. Basically there is no point even TRYING to fight a daedra in it's plane. This could be solved b having them fight over a pocket realm,( a small world, basically like the size of the Imperial city but in world-form)

2: Dagon isn't just plain destruction. He is the Leaper King, forced by Alduin to not Leap anymore but rather destroy the remnants of previus kalpa's that he had hidden in an attempt to, eventually, kill Alduin with so that one kalpa would last for ever. I doubt that Sheogorath himself is one of those peices, although a pocket realm could be made from one (and actually then you wouldn't have to make a pocket realm, you could take your favorite place in the game and have it floating in space and say it was the Leyawin – for example – from a few Kalpa's ago. But you might have to chage whatever you use to be more futuristic (Kalpa's don't end until at least the year 911 of the 5th Era.

I don't think it would make much sense for on Prince to invade another's realm. I suppose they could. I just don't know why they'd want to. A Prince is his realm, and a Prince can't be killed, so what would be the point of trying?

I can think of a couple of reasons. For amusemant. To test their minions against a rival's minions. To prove their superiority over their rivals. To acquire something that they desire. Or perhaps for reasons not known to mortals.

This idea of Daedric warring is given in my favorite and most over-cited out of game text, http://www.imperial-library.info/node/2234.

Dagon invaded Nocturnal's once.

When?

Battlespire

My reasoning is that the CoC/Sheo is not yet fully in control, just learning the ropes, so is temporarily vulnerable. Dagon is not invading for any reason but revenge, to punish the CoC for thwarting his Tamriel excursion, (and because he enjoys destruction for its own sake). He expects to trash the place a bit, and then leave, not take over. I doubt that he feels insecure, although the CoC has won out every time an Oblivion Gate needed to be closed (and the Sigil Towers were in Dagon's realm, weren't they?), but it can't hurt to put the new upstart in his/her place.

To stop the invasion, Haskill offers a truce, by your order. Dagon's protonymic can be scried from the Ohgma Infinium. Off to placate Herma Mora. The protonymic is the holy cow, of the Temple of Mara. Sheogorath must lead the cow to New Sheoth and give it to Haskill, for a banquet in honor of Dagon's victory. When Dagon eats the holy cow, he asks for the name of the dish, to which Sheogorath replies with his protonymic, and Dagon disperses from the Shivering Isles, to the Waters of Oblivion.

Thus is recorded the Seventeenth Accord of Madness.

Just an idea.