No, this is a "world expansion update", so it can be played at any time in Book 2. Basically it's about extending the sandbox, so there's always enough optional things to do.
If you've already finished Act IX, and ONLY want to play this content, you can load the ActIX End automatic save; Emily will welcome you back to Camelot. You can then play as long as you want... once Act X is done, it'll ask you if you want to return, and it'll set your savegame up to be back at the opening of Act X, though you keep any stat increases, updates, resources etc .