A Hexenwolf (or Hexenwulf, pl. Hexenwulfen) is a person transformed or being able to transform into a wolf-like beast through the employment of a magical talisman provided by another being. It is one form of lupine theriomorph.
Another name mentioned for them by Bob is "Spell wolf". The name "Hexenwolf" / "Hexenwulf" seems to be of Germanic origin, and from contemporary German would translate to "witch-wolf" or "witches' wolf".
Unlike a werewolf, hexenwolves do not transform through use of their own magic but through someone else's. To transform, a person makes use of a talisman, which can be put on and activated by certain magic words, instantly transforming its wearer. The talisman is usually a wolf-hide belt, but sometimes a ring or amulet. It is usually first acquired by a person making a deal with a powerful sorcerer, demon or devil, and receiving the talisman in turn.
When using the talisman, the person is physically transformed into a huge wolf-like beast. At the same time, the talisman provides an anchor for a "spirit of bestial rage" from the Nevernever, which wraps itself around the human personality. The spirit provides an insulation to keep the person's spirit from being destroyed, as well as helps control the physical wolf-body. Through this protection, the person's human intelligence and reason remain intact; at the same time, the person loses their human inhibitions, making them prone to act on their conscious and unconscious desires without doubts. This is at least partly brought about by the presence of the spirit, which would be too strong to control for most people; the spirit would influence the wearer, suppress their conscience, and even control them more fully. The Hexenwolf's personality, although initially protected from the transformation's effects by the presence of the talisman's spirit, will continually erode and be destroyed over time. The use of the talisman's power is further described as addictive.
Historically, the Christian Church has been strongly opposed to hexenwolves, burning a great number of people at the stake for this offence. The White Council strictly forbids the magical transformation of another being because of its harmful effects, making hexenwolves highly illegal by the Second Law of Magic.
In the series
In Cold Days, the Summer Lady, Lily, implied that the Contagion—later named Nemesis—corrupted Harry Dresden's first three big cases: Victor Sells the Shadowman, Phillip Denton and his hexenwolves, and Leonid Kravos The Nightmare.