My version:

Reference images:

Analysis: I'm not going to lie, I can't stand the Mord'Sith, and I find their hairstyle kind of ridiculous. Nevertheless, it looked like it would be fun to try. The hair is tightly pulled back from the face. A large braided bun sits pretty much on top of the head, with one braid coming out of the middle of the bun. This braid is tied off a few inches from the end, and hair is wrapped around the tie to cover it. In the show, much of the hair is very obviously fake, going by length/thickness of the actresses' hair in other shots, but the style is indeed doable with nothing but one's own hair, if your hair is between thigh and knee length and quite thick.

Directions: To get the ridiculously high bun, I flipped my hair upside down and put it into a high ponytail, combing the hair smooth. At this stage, if you wanted this to really hold, you'd probably want to use gel or hair spray to keep flyaways down and get the plastered-down Mord'Sith look, which I didn't bother with. Then, with your hair still flipped over (I bend over and let the hair hang in front of my face), separate the hair into three sections. Braid each section, but not too tightly (you're going to need every illusion of volume that you can get for this). The center braid will be the one that hangs down your back. I tied mine off with a small elastic, then used a piece of hair from the tail to wrap around the elastic and cover it. Coil the other two braids around the center one, forming a high bun. Pin the braids as you go, setting the braids on their sides so that they'll be "taller" rather than letting them lie flat against your crown. If your hair is really long, that's it; if your hair is shorter or thinner, you may want to use some fake hair around the ponytail to make the bun larger, or you can wrap the braids around a rag, covering the fabric with the hair, to make the bun look larger.