They refer to it as a domain from time to time, but they also refer to other mundane places as domains as well. They never use the term of Domain of Dread.a strange realm in the Shadowfell.
There's even a character that would fit as the Darklord: Zirithian, the drow vampire. However, the term Darklord is never used in the book.
So with all that in mind, why would I even think its a Domain of Dread? This excerpt right here:
It reads just like how I'd describe a 4e Domain of Dread.This strange realm of the walking dead exists within a splinter of the Shadowfell. Legends speak of dark realms within the plane that come into being under the influence of overwhelming evil or vast concentrations of magic. Deadhold is one of these...