Kitsunetsuki is one of many abilities possessed by Kitsunes in the novels series used as their form of possession. It is a power that can be used by kitsunes to influence the people around them and affect their surroundings.
Kitsunetsuki literally means "the state of being possessed by a fox." The fox, or kitsune, was believed to enter the body of victim -- typically a young woman -- beneath her fingernails or through her breasts. In some cases, the victim's facial expressions were said to change in such a way that they resembled foxes.