I also agree that it is two, or more, pieces to the outfit. You can do the under portion either all as one, doing a strapless dress, or you could make a tube top separate from the skirt. This way you dont have to worry about the weight of a full skirt potentially pulling down your top.
The outer tunic, if you wanted a pattern to work from, is really similar to this:
You'll need to modify the neckline a bit, and substitute the super long sleeves, but the tunic seems to be a decent jumping off point. And, bonus, that series of patterns has some excellent skirts and dresses that could work for this.
Also, the trim around the edge is likely going to be costly, but, I've found that http://www.mjtrim.com/
is really excellent for their selection and variety of options. And they ship amazingly fast.
Good luck! I can't wait to see how this turns out - that costume is absolutely stunning.
Convention Line-Up: Dragon*Con 2015
1950s Celestia (MLP:FiM), Undead Death Knight Starter Gear (WoW),
Lorna Dane Serval Industries (X-Factor), Adult Loki (Loki: Agent of Asgard), 10th Doctor (Doctor Who)