Mostly curious as to is the Dark Brotherhood has an advanced enough network where the people in Skyrim would know of and be able to communicate with Cyrodiil's and vice versa. I'm asking because I'm writing a fan-fiction that will involve them, but don't want to create something that wouldn't be true to the consistency of the "canon".

For example, if I stole from or killed a DB member, would all of the places in Tamriel be notified? If so, how? Just some overall info on their efficiency would be nice. Thank,s you guys are so smart .

Also, do the daedric cult people have an actual name? And some info on them wouls be even MORE appreciated then the aforementioned request.

At the time of Skyrim, the Falkreath/Dawnstar sanctuary is the last one on Tamriel.

If your setting it in the 3rd Era then the Sanctuaries probably keep in touch via letters.

Cicero's Journal shows that the sanctuaries probably interacted via the Black Hand with the Black Handed linking all the sanctuaries together.

I'd imagine that letters are the primary medium of communication, though depending on which theories you ascribe to the Dark Brotherhood may utilize more mystic methods to transmit orders and information in emergencies. Also, the Wrath of Sithis is invoked every time someone defied the Tenets, back when the rituals were being regularly performed. Perhaps the ghosts of the dedicated slain played some role in relaying messages?