Let me see if I understand this properly...

So, before the Oblivion Crisis, the Dragonfires created the Barrier that separated the realms of Oblivion from Nirn

And its because of this the Mythic Dawn were assassinating Uriel VII and his Heirs, So No one could relight them and once the Barrier dissipated Mehrunes Dagon could Enter Nirn and concur it

then, at the end of Oblivions Main Quest, the Barrier finishes dissolving and Mehrunes Dagon pretty much leads the Assault on the Imperial City.

As such, Martin, realizing that now the Barrier was gone, and that relighting the Dragonfires will no longer work, he gets the CoC to escort him to the Temple of the One, where he shatters the Amulet of Kings, and becomes an Avatar of Akatosh, and banishes Dagon back into Oblivion.

Now, Apparently when the Amulet Shattered, it mixed with "The Dragons Blood" (as he says in Oblivions Epilogue), and the Statue the Avatar of Akatosh became now serves the same Purpose the Dragonfires once did, as such sealing Oblivion.




Now, heres the Part I don't Understand..

in Skyrim, we encounter many Daedric Princes within the Mortal realm

Hermaeus Mora makes a small appearance when he kills the Shaman in Dragonborn
Sanguine takes the form of a commoner and challenges us to a Drinking Contest
the Beggar who Starts Sheogoraths Quest obviously travled to Skyrim from Within the Shivering Isles
we once again meet Barbas (Clavicus's Hound)
And Nocturnal Appears Infront of us on Numerous Occasions.

we even Travel to Hermaeus Moras Realm, as well as Sanguines


so does the Statue not work the way it was supposed to? did it only seal the Deadlands (Dagons Realm), because if Oblivion really had been sealed when Martin became an Avatar of Akatosh, you wouldn't think travel between Realms would be quite this common, and it would still be possible for Mehrunes Dagon to once again enter Nirn and continue his Campaign...

They arent there in any power which could pose a threat.

In the form Sanguine was in, he was just as vulnerable as any other person (and he wasnt manifested, more like an apparition), compare this to Dagon who was trying to fully manifest onto Nirn with an army.

The seal which Martin made/became wasnt a watertight lock, just something to keep out the big nasties. People can still summon Atronachs (which are daedra), but they cant open a portal for the armies of Bal to march across the land.

Sanguine and Mora were visiting, not materializing in battle form ready to wage war and summon a hundred Daedra for conquest. The Princes have always been able to visit Nirn in the form of avatars of themselves. Dagon took that to the next level, and that was because the barrier was pretty much gone at that point.

As Haskill put it, the Daedra can manifest portals and themselves into a realm so long as they don't pose a threat to Mundus. Mora manifested himself in close proximity to his artifact, and once again shows up when Crag-Strider willingly opened the book. Beyond that, the Princes speaking through their worshipers or shrines is not new.