Dark Shades are creatures with an ability like no other creature in Skyrim. You conjure one Dark Shade, that Dark Shade conjures another, and its Dark Shade conjures another, and it goes on forever (only while in combat). Kill the main one, they all vanish one by one.

Release all of your negative energy and watch as darkness itself multiplies.

This is not my entire mod, this is but a part of it. I uploaded it simply because this creature is that awesome. Try it for yourself.
You can buy it from Farengar (the court wizard up in Dragonsreach). It will be very, very expensive, both in gold and magicka. Changes will be made when the mod is completed.

Trivia:

Every Dark Shade conjures another Dark Shade automatically at the start of battle, causing them to expand into an army.

Dark Shade's levels can go from 1 - 100. Their strength will increase as you level up.