Whose demesne? A demesne is a world or pocket reality belonging to each of the Council - it's not one place.
I mean like I can't access it can't find event to access them
You gain access to the various demesnes once you open the Nexus. It's a story event so you can't miss it - just keep playing, and pursue both the Fey and Eldritch questlines to get it as soon as possible.
