Durza

Eragon was startled by her sudden vehemence. “I don’t understand. Brom told me that Shades were sorcerers who used spirits to accomplish their will, but why does that make them so evil?”

Angela shook her head. “It doesn’t. Ordinary sorcerers are just that, ordinary—neither better nor worse than the rest of us. They use their magical strength to control spirits and the spirits’ powers. Shades, however, relinquish that control in their search for greater power and allow their bodies to be controlled by spirits. Unfortunately, only the evilest spirits seek to possess humans, and once ensconced they never leave. Such possession can happen by accident if a sorcerer summons a spirit stronger than himself. The problem is, once a Shade is created, it’s terribly difficult to kill. As I’m sure you know, only two people, Laetri the Elf and Irnstad the Rider, ever survived that feat.”

Mandrake root and newt’s tongue, Eragon