The fabric is polyester Silk Essence which sews and drapes beautifully. I used McCall's M3789 (all purpose Wizard and Jedi robe pattern) as a starting point. The wizard robe pattern calls for waaaay too much material, in my humble opinion, so I went through several tries to get the Snape-y, batwing, flowing effect.
Both front. back, and sleeves feature many, many pleats.