Durza

What are the spirits consisting shades? And since magicians can control these spirits to create a shade why can’t they control them after they have formed the shade?
Spirits are matrices of self-organized, self-aware, cohesive energy. In other words, they’re thinking balls of lightning. Or light. Or heat. Or however you want to define energy. When a magician summons spirits, he or she binds them with spells and with the strength of their will. If the spells are badly worded, or if the magician’s will falters, then the spirits can break free and take control. Once the spirits form a Shade, they’re immune to that sort of outside interference, since the spirits are now bonded to the flesh of the person they’ve possessed.

http://www.shurtugal.com/2010/11/16/random-buzzers-qa-with-christopher-paolini-part-3/