So, does anyone have any idea if Mehrunes Dagon invaded Akavir (or, come to think of it, Pyandonea, Thras and the various other regions outside Tamriel as we know it) during the Oblivion Crisis? I suppose taking Cyrodiil was the top priority, which in turn means that the other provinces would have to be neutralized, but is there any chance that he attacked the rest of the world, to?

It's possible, but I think not. Mankar Camoran referred to Tamriel as the realm of Lorkhan, not Nirn, which leads me to believe the Imperial provinces were the focus of the attack.

I wouldn't put it outside the realm of possibility for a guano-for-brains conspiratorial hokerr to use the "wrong" terminology.

Nothing is none, and I wouldn't go farther than that, as there is absolutely nothing else on this, let alone anything at all about Akavir, save for 2 books since Morrowind.

Nope, but on another board on this forum, I'm planning on arbitrarily ruling that a rogue Daedra attacked Akavir during the Oblivion Crisis, to be thwarted by the Mighty Morphin' Akaviri Power Rangers.

Mythic dawn agents had to open the gates. Unless they were some in akavir no gates would have opened there.

They wouldn't need to be mythic dawn agents, they would just need to be Dagon worshipers that could conduct the proper rituals.

Tamriel was the focus of the attack. For it's the continent with a province that holds the key to relighting the Dragonfires and banishing the daedra.
At any rate, Oblivion gates opened within Akavir would be severely terminated. Tsaesci forces would drive into the realm and slay anything inside.

Hmmm.. a bit off-topic but if one went into the Deadlands via a portal opened in Tamriel and then another opened in Akavir, could one access that?
Like by finding the connecting portal within the realm and walking through to access Akavir? has there been any reports of a similar event possibly?

Reports in the lore of one person going from one gate in the deadlands to another? not that I can think of. (Though Sul used various passageways through the realms of oblivion to get to Vvardenfell without having to step foot into any other part of morrowind, and a similar method of travel to Sul's was used in the in-game book "The Doors of Oblivion") However, If my memory is intact, I do recall seeing, on rare occasion, other oblivion gates besides the one I entered into. I see no reason why one couldn't have used the Oblivion gates to get from one part of Tamriel to another, though it would be a hugely impractical means of travel.

It's as impractical as Vaermina's Torpor, which is still quite a popular method of travel to the people of Tamriel even though of it's dangers.

And I was just wondering because a tamriellic province which had a Daedric Prince's blessing, could use the daedric realm travel to invade another continent easily.

And that's different than to what happened in Tamriel... how?

I don't see any indication that the Tsaesci are more powerful than any Breton or Nord in the lore of the species, even if they are longer-lived. (I also ascribe the the theory that when the book mentions the Tsaesci "Ate" the men of Akavir, it actually refers to cultural, not physical/oral, assimilation.)

Dagon acted in the Crisis in his God of Ambition role. His Ambition was to topple the Third Empire and replace it with the Empire of the Daedra. He almost succeeded.

Hmmm. You know, judging from the account from Rising Threat it was the fall of Crystal Tower that allowed Mehrunes Dagon to manifest onto Tamriel. The final levy to break if you will before the Tide of Oblivion rose and flooded. Before Akatosh breathed fire and evaporated the flood.