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.
Party Pirate Pinkie Pie (MLP:FiM) 100%
Eris Discordia (MLP:FiM) 100%
Convention Line-Up: BronyCon 2013 - Dragon*Con 2013
Eris (Female Discord, MLP:FiM), Casual Gala Pinkie Pie (MLP:FiM)
The 10th Doctor, blue suit variation (DW), Queen Mab's Party Dress (Dresden Files: Cold Days), Party Pirate Pinke Pie