A thrall is a human person put under some kind of mental control.
In Summer Knight, Elaine Mallory describes what it was like. She didn't know it was happening at the time. She didn't have the ability to think clearly. Justin told her that he just needed her to hold Harry down long enough so he could explain things to him and everything would work out. Justin used her trust of him against her. 
They are divided into rough thralls, whose free will is completely removed and essentially behave like machines, and fine thralls, which may not know they're enthralled. It takes a subtle hand, a lot of time and a certain amount of empathy—sort of like what Justin DuMorne did to Elaine Mallory. They may be freed or recover over time, unlike Renfields.
Creating a Thrall is against the Fourth Law of Magic: "Thou shalt not enthrall another".(reference needed)
In the seriesEdit
- Main article: Summer Knight (novel)
In Summer Knight, Harry Dresden and Elaine Mallory discuss their past with Justin DuMorne. Justin had already made Elaine his thrall, then commanded Elaine to pin Harry down while he tried to invade Harry's mind to enthrall him.
- Main article: Blood Rites
- Main article: White Night
In White Night, Harry Dresden and Carlos Ramirez are escorted by Lara Raith down a red carpeted pathway leading to the Deeps. Along the path are human thralls, kneeling, draped in white silk kimonos and tied with a single strand of white silk around the throat to poles. These thralls are intended to be "food" offerings to the guests of an impending White Court meeting of the different houses. Along the path with the human thralls were small faeries in cages forced to be lights for the path.
- Main article: Aftermath