During the events of Oblivion, I seem to recall a note from one of the Mythic Dawn cultists that implied that in order to open an Oblivion gate, there needed to be somebody in Tamriel with a sigil stone who could "anchor" the gate in place. Now, given that the gates opened all over Tamriel, that would seem to imply that the Mythic Dawn was operating across the breadth of the Empire, and with some reasonable level of organization. Correct me if I'm wrong about how the gates operate, but if true, then how would an organization like the Mythic Dawn, operating over such a large area and with such a destructive goal in mind, stay secret from the rest of the world? Surely you'd think that somebody would have caught wind of what they were trying to do, yes?

Its been a long time, but IIRC, Uriel's death made the requirement of a Sigil Stone being utilized on Mundus moot.

The gates in this scenario, are temporary affairs, and likely are not the method the Mythic Dawn utilized. The Sigil Stones are being harnassed within the Deadlands itself, so my guess is that Dagon opted to just start punching holes in Mundus once White-Gold went down. Between the Mythic Dawn trying to hone in on Martin, and following Camoran in a mass suicide, I don't think they have the man power or resources to try and open each gate individually.

I do think the Mythic Dawn may have operated elsewhere, though, most likely trying to further Camorans ambitions in their own right. However, I'm more convinced that they're sleeping agents like God-Hater, and aren't really sizable enough to ultimately impact anything. Their infrastructure certainly wasn't vast by any meaning of the word, since by the time of Skyrim, they seemingly have died out as an organization.

"Jearl -

The Master was pleased to hear of your activities outside of Chorrol. The more gates that we open, the nearer we are to the glorious Cleansing."

It only needs one person and that person can obviously open more than one gate .Two or three members per city would be enough.

But if Dagon was just brute-forcing portals into Tamriel, then why does each gate still have a sigil stone keeping it active? Doesn't that imply that the stones were still used to create the gates in the first place, even if the fall of White-Gold allowed them to stay open indefinitely? And if, theoretically, Dagon was able to create openings into Tamriel at will during that period, isn't it conceivable that a Prince like Molag Bal would have attempted to capitalize on the opportunity as well?

Fair enough, though this might only be still exclusive to Cyrodiil. By the time the Oblivion Crisis came in at full swing when Dagon himself emerged, I don't think mortal agents are needed at that point anymore.

Even so, I don't think the Mythic Dawns power structure was still vast enough to actually coherently span across Tamriel in a significant capacity. Otherwise, they likely would not have rotted away as quickly as they seemingly have.

While I generally agree that the Mythic Dawn was maybe never that big, it is worth consdering that the next time we hear of them is 200 years later. With apparently numerous groups like the Vigil of Stendarr hunting them, in addition to the Blades and in all probability being hunted by most local governments in some way, even a large organization can be killed off. Especially since we don't know exactly when they died out. I mean, look at the Blades. An extensive network of spies, most of whom were professionals of various trades, and they are near eradicated in the Empire after just 26 years of running from the law, due to the Thalmor being dedicated and capable.

Many small, largely independent cells of sleeper agents(like the family in Dawnstar) who are aware of how to open gates once the Emperor is dead could account for the gates appearing all over Tamriel. Cyrodiil is certainly not unique when it comes to housing secretive Daedric cults. They seem to be spread out over the whole of Tamriel.

Another possibility is that, since Dagon is lanching an invasion, that he answers his worshipers in the other regions and gives them the mission to open gates. This both keeps the Mythic Dawn group small and lets Dagon have his gates all over the place. Probably a mix of both.

Their leader was dead and Dagon didn't have any interest in them anymore.How long would they continue ?And Silus Vesuius family was only found to be part of the cult long after the crisis a lot of them probably went into hiding or those that weren't found out just stopped being part of a dead cult.The really fanatic members probably died during the crisis or shorty after.